I'm not sure BLM support is "mainstream". Home Depot hasn't said anything official about BLM to my knowledge, and you hear loud pro BLM people saying they should be boycotted [1], but I think they're doing fine. Their stock price is as good as its ever been.
I think what we're seeing is a case of "minority rule" [2].
I think my point is more that people who actually change their behavior based on whether or not a company explicitly supports BLM is not a majority. It is probably a small minority. If that 57% actually boycotted businesses not supporting BLM, Home Depot would change their tune quickly.
Companies prefer to not be in the news. So what they do is seek out the mainstream and align itself to it.
BLM was not mainstream 3 months ago, so companies remained quiet.
Now, not showing support for BLM runs the risk of unfavorable PR. So they put out a low risk statement.