It forced unnecessary retcons to characters’ history
When the main character of the show, Michael Burnham, was introduced, she was introduced as Spock’s sister. Now, if you stopped watching Star Trek prior to the 2009 films, you may ask “Wait, Spock never had a sister” and you’d be correct. He didn’t. They retconned Spock having a secret, adoptive, human sister, who for some reason acted just like a Vulcan. Even though she wasn’t.
One could argue that’s Cultural Appropriation and is just another reason you could add to why fans didn’t like her. But her arrival changed up the lore of Spock unnecessarily and all just to cater to a new character that no one really got attached to.
The disruption to Spock was only done to make Burnham seem more interesting and worthwhile as a character, and in doing so, they made fans resent the character. They saw the retcon as nothing more than an attempt to make this new character more interesting at the cost of an older character.
Burnham would’ve found much more of a fan base if she had an original backstory, one that didn’t require decades of storytelling to be ignored in order to bolster this new character.
And the less said about her name the better.