Well, I can speak for my medieval patch. I haven't worked on it for quite some time now, but it is by no means dropped. It just means that I've cast my affection on other things for the moment, including my african epic! -which should be coming along a little faster, because I'm working on this every once I get the opportunity.
It's hard work to do these things, especially if you want to go all the way, and really make it worth doing it, historically interesting, looking good graphics-wise, and playable/challenging/entertaining etc. Making all those graphics, setting up the rules, thinking and scheming all those nifty details that make your mouth water... Takes a lot of time. I read that Bebro spent years setting up his SubWar scen. Well, I'm not willing to spend so long time on this, but I can see what he means.
For me, these new projects are very different from my first scenario, where I just went about my business without too much worry, and just a minimum of research.
Now I want to really push the limits, and among other things, this takes a lot of time going into research, studying old maps, reading all kinds of strange books etc. And perhaps getting a little caught up along the way.