Hey as some one who has lurked here for mods and to view, figure i'd finally make an account myself. How i get myself into Civ 4 was mostly word of mouth, from a buddy quite a few years ago. The problem is compared to most newer games it's very hard to get into for a few reasons, looks difficulty etc. But one big problem is a lot of people just don't want too (Maybe can't) enjoy sitting down and playing games for a long time. I personally stopped playing this when i started because i thought i was a horrible player because i couldn't beat Monarch on my first try. A few month's later i booted it back up and improved etc. It's a tough game to learn, but once you get the feel for it. Well it's a pretty great feeling, even losing or winning people could just dogpile you and it's game,
I myself love that, most do not in the normal game market. As some one who's in there early / Mid 20's I was surprised at how many people just don't make time for games anymore. The only other game i play is WoW classic / Vanilla etc back in the day when i was much younger mostly because i don't enjoy games with endings. An example is, i just played Divinity 2 Great dnd Strategy type game. But once i beat it well ain't much point in replaying it, besides for more story options etc. I've had the same problem with FPS Before multiplayer came out besides a select few games.
Civ 4 isn't really a game about winning as much, sure you can rush rifling / Military tradition and just rapidly conquer everyone. But that gets boring fast, and why would you want to win that way over and over again? Does it fit the narrative of your civilization? Can you do that? Are other people to far etc etc. Even in maybe you're best games you can look back and find ways to improve in a way it's sorta a timeless game once you find yourself in a spot you enjoy. Personally once you get to about 1800's era tech for me, the game just feels like a slog, and it's tough to lose or come back at this point. But no doubt others enjoy the end game and dislike the start.
It's also very tough to find games like Civ 4, I've tried, never was big on Grand strategy's myself. I think the only real strategy games i got into was, Warcraft 3 and Fire Emblem. (If you'd call em that.) and Chess.
I think history is also a big factor as a fan of it, it most certainly opened up for me personally more people to learn about and such. Not being the smartest guy in the world it always shocked me that most people who do get a higher education then myself, seem indifferently uncaring or lack any knowledge of a good chunk of it. Heck i remember talking to some people at my local college who didn't even know who De Gauile was. Or Hannibal. There's a much better i don't know. Want to get into a game as a character you'd want to play for sure.
But i also think people just enjoy more colorful games, to me the graphic's don't look bad at all but most people run pretty beefy PC's now and days, although to be fair games like minecraft are huge and they don't look like a game straight out of a moive. I think people are more use to a hands on approach as well, for example being told what to do. Why / how / when etc. To some that's more welcoming then, here's a settler i hope Barb axemen don't spawn next to me as i learn why building roads to the sea doesn't let me make ships!
I think age is also a huge part without a Re-master people just won't seek it out, unless they know about it already or word of mouth. People just naturally don't look to the past for better games, and that's a shame because there are aton of amazing games from early 2k, 90's etc. One i always like to name is planescape torment, and the multiplayer is slow. Very slow, but i'm also an oddball in a sense i only enjoy marathon games. But plaything that on Multiplayer is rough everything takes so long ending turns. NPC RNG etc.
But that's just my two cents i only really dump a ton of hours into this game and O.G WoW so who know's. I just guess it's a mentality difference. After 8 hours of work, not everyone wants that super challenging world conquest game, but at the same time they won't put it on settler to just steam roll the world. They'll play another game etc.