It's because there is a lot of people who do not want to be painted as a villain for essentially being a white male. Which is not what blacklivesmatter or believewomen does but the phrasing makes people feel uncomfortable and part of the problem when they aren't.
I am not saying I am one of these people, once I understand the movement I get it but I sometimes hate the wording and phrasing of these movements cause I know people will get confused and feel they can't be part of it.
Literally only a racist would think Blacklivesmatter means all white people are evil