Because it worked. It stirred up controversy, it got YouTube videos, it got clicks. Even everyone who complains about the writing and not the factions themselves or the characters involved, it’s exactly that kind of controversy that got attention.
For Blizzard, in regards to the Burning of Teldrassil, there is no such thing as bad publicity.
Which really ought to worry everyone about the future, because if they think it worked once it probably means that they will think it will work again.