I think accepting would've worked. Implying that some demographic is bad makes the movement lose support, the only people who benefit from it are people trying to gain status within the movement. (Edit: that answers my question above.)
It's not the idea that being white is acceptable that's being rejected though - most of the people doing the rejecting are white - it's the idea that white nationalism has anything to do with 'being white' or that objections to it have anything to do with aversion to whiteness. It's a bit like seemingly innocuous positive statements about 'life' or 'choice' suddenly become minefields if they're used almost exclusively as campaign slogans by one side of an abortion debate.
The whole structure of the "it's ok to be white" thing is to (it it were accepted by the left) to start delegitimizing the attempts to build alternate power structures that aren't available to non white people. For instance, the NBER found that literally changing a name from Tyrone to Chad on a resume increased job callbacks 3x, and stuff like affirmative action is designed to address exactly those systemic inequities. Normalizing "it's ok to be white" was designed to attack those head on if it was accepted.
You don't see that half, because the left didn't take that side. There was no winning either way.
