Mostly because Toady has also stated in the past that he's not sure how to go about it. His opinion of UI design, last I heard, was about the same as his opinion on his ability to make proper art for the game. With that in mind, the solution seems to be to do the same general thing, only with interface placement rather than with graphics.
That said, if he decided to go for an interface overhaul on his own that's acceptable too. I linked the most important post in the thread though in the first post, new bolded edit. Read through it. It actually breaks down the reasons why it should be dealt with, either as abstraction or fundamental systemic changes to individual systems.
Also nothing specifically wrong with the raiding interface, was just a further example of yet -more- features being added and thus adding complexity.
Read through the opening again, I recall it from when it was first brought up. It seems that Toady's donations have actually increased in the past 3 years, so the urgent need for new players, seems to have not been true.
The main reason for this would be to let people build new interfaces. Which is nice and allows an opportunity for monetization, which everyone likes, but isn't likely to have any impact on whatever improvements Toady makes by himself (whenever that may be) since nobody can ever agree on what a "good" interface is (and those that make a good attempt, may turn out to think no-one plays adventurer so don't bother with that, etc, etc). Not sure if it would even help locating bugs, would it? So low priority probably. At least compared to what he's doing right now in tackling a big chunk of the army arc which actual players have been looking forward to for ages. Far more exciting (in my opinion).
I'd seriously watch the interview for his current thoughts on the interface though. "In the past" has no meaning when last week, he said "in the past, I was wrong".
Also, "proper art"? It's a text and ascii game. It already has "proper" art. And the need for a tileset extension was mentioned a few hours ago. So, more "proper" art coming. That's a completely different issue to the UI.