They didn’t though. Specifically the humans from Lordaeron did that. The Alliance as we know it today is primarily Stormwind on the human front, though a good number of high elves are part of Dalaran. The other Alliance races like dwarves, gnomes, night elves and the like had nothing to do with what happened to Kael’thas and his followers.
Garrithos wasn’t particularly nice to dwarves, either.
Except Blood elves joined the horde over wrongful persecution by the Alliance when Kael joined the Burning Legion.
Blood Elves were still members of the Alliance until then. They needed support when they lost Kael and the Alliance instead persecuted them over the whole fel thing.
Garithos was a warlord who led remnant forces in one region. The Alliance essentially didn’t exist as a uniform organization. The fact that people think Thalassian civilization would turn its back on humans because their prince chose to follow the wrong one into battle is ludicrous.
There is nature and there is nurture. Sometimes it’s hard to tease apart the differences between different personality types. Were they wired that way at birth? Was it because of their upbringing? Or was it some of both, as people who are by nature a certain way tend to marry with other people like them.
In any case, there are people who feed on options. They take strength from exploration of what they can become by taking advantage of available options.
And there are people who feel that options are a bad thing. They allow people to stray from their own idiosyncratic One True Path™. They tend to be autocratic in nature. They value obedience as a virtue and preference for exploration of the unknown as a character flaw. Or at best, something must be earned, yet still be tightly restricted limited for those who have earned that privilege.
So one group enjoys playing all parts of the game and has fun doing so. The other frets that people are playing the game wrong and should be stopped by removal of options. They don’t play the game for fun. They play to enforce their concept on others who won’t be happy if they are forced to play in a much smaller game with next to no options.
A certain amount of stasis is necessary in order to preserve the integrity of the game.
That is not to say that sometimes a change isn’t appropriate for a number of reasons (ie changing RL norms) but the reasons need be be more meaningful than ‘But I waaaaant it!’.
Most players are not against this. General Discussion is full of clowns that will comb through pages and pages of broadcast text and datamined information JUST to find something to complain about.