I think a more pertinent question is - why would we want DF to gain "mainstream" popularity?
Toady One appears to making enough from donations to support himself comfortably and continue development at what is a pretty impressive pace for one single programmer. Mainstream appeal would mean more money for him, yes, but no appreciable increase in development speed, unless he started contracting out the coding work (which I really would not want). And consider what a "mainstream" fanbase would do to the game- less focus on complexity and versimiltude, more focus on "game-ness" and graphics, which I personally think aren't in need of improvement.
I guess I don't understand what's so bad about the status quo.
This man has a good point. Mainstream acceptance would probably lead to pressure on Toady to change things about the game that he, and most likely the majority of us that have appreciated this game for a long time, wouldn't want to see changed. I'm not sure how that kind of pressure would really affect the good developer since I don't know him personally, but I can't imagine it would be a pleasant experience.
On the other hand, I would love to see DF appreciated on a wider scale, and I most certainly wouldn't mind having a built-in graphical tileset. Maybe we could have some kind of middle ground, like, a built in launcher that comes packaged with the game that includes a graphical tileset, kind of like the way LNP is packaged? Certainly something like that wouldn't be difficult at all, especially considering the community has pretty much already built it?