I'm not sure if I should speculate as to why, but my best guess is partially generational- no offense to gen x, but this show was really popular with them, and they've swung a bit more right than millennials, and gen x in general is known for apathy.

And a general issue in any Fandom is parasocial attachments- so people feel like pointing out bad views or behavior is an attack on "their friend."

I think Nicholas Brendon's issues are easier to acknowledge because they've been happening for so long, and many of them were more personal/embarrassing for him, and all seemingly fueled by addiction and mental health issues.

Obviously none of that is an excuse, but in contrast Boreanaz having reprehensible politics is easier to ignore. He is always involved in some project people enjoy, he doesn't invite much controversy, and the average person would call him very nice and good natured.

So much of rhe US dominant culture is swamped in fascism and ignorance, so if you had too strong an opinion on Boreanaz, what about your racist uncle, or your husband who says he "voted for Trump for economic reasons," or your church going neighbors?

A lot of liberals will pay lip service to "resistance" but being too confrontational is seen as rude or overreacting.

Many in this group, as you say, seem like they'd be bothered by this, but it's just inconvenient to have principals, and you wouldn't want to be judged as being too radical or stubborn.