Originally Posted by
Fistfighter
Because at some point you ran into something like MoP, where Druids got Metamorphosis, which was a nightmare to balance and Mages got Alter time, which was underwhelmingly boring.
Or WoD, when Hunters got aspect of the fox, which was fundamentaly too OP to exist and was taken away right away, while Mages once again got an underwhelmingly boring Amplify Magic.
What I am trying to say, it's hard to come with a new interesting and usable abilty for each class, and most of the times they will end up repeating themselves, or being forced to create abilities just to pad the numbers.
And what happens when the ability just.. doesn't work, removing it seem rather harsh, considering the rest of the classes get to keep their new shiny toys.
The borrowed power of (especially) Azerite armor was just enough new flair, while still remaining fresh and and interesting to use.
And don't get me wrong, I am for adding new base class abilities, IF they really fit, and not every class really needs or warrants a new ability each expansion.