they didnt introduce DH because they didn't feel like it actually
from the interview
How long was the demon hunter class in the works and have you wanted to implement them for a long time or only recently?
A: Since we played WCIII we have wanted to implement and play them! Has been discussed during BC but mostly during the last year and a half.
the exact wording from the interview "we wanted to impliment them since we played illidan in WC3. Like way before TBC." then the other guy "yeah that was one of the things from my first month w??? (audio screwed up) was 'hey can you try to figure out some way to get the demon hunter class into the game' and it uh as it was the timing didnt really work out because the burning crusade was the first opportunity and then we just waited and waited until we were like 'finally' legion is there this is the time"
so contrary to your almighty opinion oh ye lord of warcraft classes it was apparently just not something the devs attempted and by the time crafticus came in it was too late