As to why they would do that, well the same reason anyone would does anything. They get rewarded by the community for features, and many users in the community probably like that the game is so arcane.
All the users that would thank them for a better UI have long ago abandoned the game.
Wrong on both counts. I think Toady said he has plans to sort of go by that Eternal Voting suggestion thingy in the future, at least in regards to what things he was already going to add that he's now going to add
first, but he does not currently, nor has he ever added features based on what the players want or what they're willing to donate. Which IMO is a good thing, because not to be mean or anything but a lot of the suggestions here are pretty retarded, and nobody should have more say than somebody else just because they have deeper pockets.
Secondly, I have never heard
anyone defend the UI, on this forum or any other. We all know it sucks, it just doesn't get brought up anymore except by new people because, well, nobody constantly brings up the fact that rain is wet either. Being willing to put up with it because the alternative is to just not play at all is not the same as 'liking' it.
Personally I would love to see the UI improved immediately, but I do understand Toady's reasons for waiting. A lot of DF's current features are basically placeholders, it's still getting overhauled and added to in such huge ways that it would only mean doing the work over and over again - a better UI for the game as we know it now won't make sense or even apply in a lot of areas for the next release, and would need to be ripped out and drastically changed again for the next, etc.