They brought back quite a number of characters because they were killed off "unfairly" and/or didn't have enough screen time.
Gul'dan in WoD, Illidan in Legion, a whole bunch in SL, including Kael'Thas and Uther.
Why did they randomly kill off Ra'den in BfA? Did Ra-den have too much screen time?
I get that Odyn essentially replaced him as the Aman'Thul's watcher, but wouldn't it be interesting to see those two cooperate or maybe compete against each other?