Well first, because it's a lie. At least it is a major exaggeration. And second because it alienates some of our own and some of our allies, making them choose between two wrongs to right.
Racial harassment is still a major problem in schools.
Maybe less often than thirty years ago. Probably more often than they once did, but if they always treated these incidents with the gravity they deserve, racial harassment in schools would have slowed to a trickle by now. The implication of the statement is that racism is solved and it is time to move on to homophobia. We mustn't choose which wrong to right. Fairness and safety aren't finite commodities we have to divide up.