Because the game still feels great and is a breath of fresh air to the genre (speaking as, foremost, a lover of RPGs). Also, not as much money has gone into this game as you would expect from a game of this quality.
Sloppiness can be fixed, what can't be fixed is a boring game (this is of course a subjective thing, personally I find that nothing new has come out of the gaming industry in quite a while) .Sure, not everything is perfect, but, with a few exceptions, the system is great, there's a great feeling of exploration etc. that I just haven't felt in many recent games. The game has a lot of at least somewhat unique features which make it stand out (excellent co-op, aside from the difficulty following your co-op partners conversations, environmental effects system).
To put into context, the last game I've played that I'd say was 'great' before DOS was Dragon Age: Origins, and I prefer DOS.
If you bought this game as a turn-based game and don't like RPGs, maybe that's what you aren't getting, I don't know... A lot of the fun to be had in an RPG (for me, personally) is the sense of exploration, being able to roleplay your characters, feeling like you're in a world that is 'organic' where things make sense (internal consistency). It isn't just about the gameplay mechanics. This isn't at all a game like W40K:DOW2 which has lackluster story and is really about the mechanics (which are of course great).
What did you expect when you bought the game?
