How the hell do you think that movement started?
It started with Colin Kaepernick, an NFL player taking the knee during the National Anthem in protest at racist police action, he was slaughtered for those actions by all and sundry including then then President and one of racism's biggest supporters.
You said " There isn't stories of the losing NFL or NBA teams being racially abused online" - so by that logic you don't count message boards as being used as a form of online abuse.
You've got to be pretty naive if you think News website comment forums are just a matter of left wing nutters vs right wing nutters, some of it is but there's a lot more than that, some of the stuff on there has nothing to do with left or right, it's just pure hatred