Yeah it's a big issue.. so many thoughts come to mind (about your last paragraph in particular) that's prob gonna turn into a wall of text sorry lol...
The short version:
1 - Civ2 & ToT back in stores and (if it happens) us doing our part to lure people here will help, 2 - Better welcome information (I'll keep working on that draft) and helping newcomers find here & make the transition to ToT will help, 3 - Promoting your excellent work around the web bringing people here will help.
The long version:
1. Yes the biggest problem is lack of mainstream availability. EA finally got off their butts a week ago and put
Alpha Centauri and all the classic Command & Conquer games on Steam (if only 2K would bloody do the same) and seeing the huge influx of returning players in the Steam forums hungry for mods and conversation has been lovely to watch (with SMAC steam, I & others have been in there posting CivFanatics links to get some of them coming here). This is of course what I have dreamed about for this place with the
getting Civ1 & Civ2 back in stores project. I personally don't need Civ1,2&ToT on Steam & GoG and none of you guys do either probably as we've all got our discs however this
community needs them and Steam in particular (love it or hate it, as it has by far the biggest customer base) as classic Civ games on the largest modern digital game stores will put it in front of countless eyes again and bring back so many players. If I ever have success on the Civ1&2 back into stores project I'll be in here very fast letting you guys know and I hope that people will join me over there in the new GoG & Steam Civ2/ToT forums constantly promoting this forum (eg every time someone asks a question, instead of telling them there, just say "you can find the answer here" and link to whatever CFC thread answers the question) luring people here. I recently did another big post run across all Civ groups on social media promoting the 2 year anniversary of the project and it looks like we've gathered a few more thousands votes on GoG which is nice but sadly no big news. I got an insider tip a year ago that there were talks going on between GoG & 2K but I've heard nothing since sadly although he did say to be patient and not hold my breath as these things take time lol.
2. I have noticed a very slow but steady influx of new or returning people posting questions every few months in the general and tech help areas of our Civ2 forums here but I don't see them coming into the league. So as Techumseh suggests it could very well be (a) the complexities of the advanced work going on here intimidating people a bit, or (b) maybe they just flat out aren't even finding here. With the former (a) I've always said MGE makes a good stepping stone to ToT due to it's scenarios tools and then once people are used to it then try ToT and maybe even LUA etc since lets face it, you know what most people are like on the internet, if it aint easy they won't do it haha, so it's up to us to try and make the transition/learning to ToT easy and if they still don't want then we still wish them well and help as best as we can. While the later (b) can be helped by SL members visiting the other Civ2 sections and promoting the SL in their answers to questions. I also have plans to construct a massive pinned welcome & information thread in all Civ2 sub sections (except the SL as you have your own) similar to
the big one I made in the CFC Civilization 1 forums that will heavily promote the SL and all the mods, scenarios, repositories, resources and guides you've all been making over the decades. I've got an early draft but it needs a lot of work still and I've been putting most of my free time into my mod projects lately as I so desperately want to finish them after all these years. Either way once I do finally put the thread up I'll need help from all you guys pointing out the gazillion things/links I will no doubt miss haha.
3. Unfortunately the community isn't really big enough anymore to ensure lots of playtesters as we're all busy working on our mod projects and so stopping work to go play another persons project is a hard choice when ones free time is so limited. So in the end if modders want their mods to be played they're going to need to put on a marketing hat and go promote their work around the web. Which sucks I know as most modders just want to create, but if you want players you gotta work for it lol. Personally I start by taking things I've posted here and posting them on all other surviving Civ forums, I also
build pages for each of my mods on ModDB and use it's blog system to promote my work there via dev diaries as they have a large userbase, I also
create videos on youtube showing off the work I've done (which I think
@JPetroski and
@Prof. Garfield do for their stuff too), and because those things are not enough alone I then share them across social media as there are many Civ groups out there across FB, Reddit, and Discord.. and all these posts and content out there all contain links back to here. As a result I've been able to build up some good interest around the net in my work so if I can just bloody finally finish and release my mods after all these years it'll be great haha. As you guys know I finally released the
Heroes of Might & Magic 2 mod here late last year but I actually haven't done any marketing yet on its final release (as I want to finish some other things it references first) so I really didn't expect much as I know all you guys are busy, yet surprisingly it's had
83 downloads here at CFC,
52 at ModDB, and
72 on Internet Archive (although that one may just be views) so I can only put that down to other site traffic and my earlier marketing efforts sharing videos and dev diary blog posts around building up a waiting audience. So I'm looking forward to seeing what happens when I actually DO share the big mod release announcement around the net lol.
Obviously in a perfect world I'd love to be out there promoting everyone's work here and I've been working for years on huge tour videos of heaps of scenarios here that will help to do just that, however because I just have too many plates spinning it's all taking forever (I suspect if I hadn't of taken on the moderator and social media position at CFC I might have finished much more of my videos and mod projects by now but oh well I enjoy the job lol) so unfortunately you can't really rely on me to share all your stuff for you sorry, so you're gonna have to get out there yourselves. I can still help a bit with that though as one of the benefits of me doing some of the social media stuff for CFC is that I've built up a huge link list of external Civ fan groups across social media with literally tens of thousands of members in them and I'm happy to share that information with anyone that wants it. Another area I can help with is that if you want your final mod release posted on the main CivFanatics news page both
@The_J and
@JPetroski have access to that area (I don't as something went wrong when The_J tried to add me) so if you PM them them a short writeup (what it's about, why it's amazing) and a nice picture and/or video they can consider sticking it on the front page as official CFC news, which then it's part of my job here to take and share across many of those Civ social media groups putting a lot of eyes on your stuff. That's for once off final releases only though as it's a busy news service, so if you want to do lots of promoting/spamming of your development and different earlier stages like I do for my mods to build up interest then you'll need to get out there on your own, which like I said I can give you a link list and am always available for tips/advice if anyone needs them.

The added benefit of bringing more people here to try your mods are that it potentially grows this community and brings in people who may want to try other peoples work too. So promoting your own work in this place has the potential to promote everyone else's too. A rising tide lifts all boats kinda thing lol.