DBM wouldn't have been necessary if WoW boss fights weren't overdesigned and their mechanics poorly presented to the player base. They are 10+ minute long fights with 20+ different unique mechanics and adds going off, depicted as tiny buff/debuff icons beneath the already small boss' nameplate, which you may or may not be looking at because you are also having to look at your cooldowns so you can play your rotation and the HP of your teammates and what is going on in chat. The fights are incoherent and you die for reasons that cannot be easily deciphered. Maybe you can spend an hour studying for just one boss, like a trial in FFXIV or a strike in GW2, but in WoW if you're PUGging you're expected to kill at least a half dozen or so bosses within an hour. It's impractical to study for all of them. DBM is the only way to somewhat make sense of what's going on and get your clears done in a timely fashion.
Contrast that with FFXIV or GW2, where the boss fight slowly tutorializes the mechanics to you, each mechanic in isolation with clear and consistent visible indicators of what is happening with clear, extremely visible indicators, before combining them all at once later on in the fight..