Originally Posted by Kardiophylax
Perhaps because some people like ARPGs? Isn't this question a bit like asking why do they keep making Wargames, FPS games, RTS, RPGs when there are so many already? How many games does the world need?
Just because a game isn't your preferred style doesn't mean it's not someone else's. Turn-based RPGs are likely a smaller niche then even ARPGs, which is what I call the games as opposed to Diablo-clone, which is the way a lot of people who dislike ARPGs prefer to call them.
There are a lot of developers who focus on one genre of games. I don't personally play many FPS games, but I don't criticize Valve for continuing to make them either.