Oh so many reasons.
1) it equates the anger of the oppressed to the hate of their oppressors.
2) it blames oppressed groups for their oppression. Bigotry doesn’t exist because people hate bigots. It exists because oppressed people oppose it. It exists because of bigots and because of privileged folks being complicit or tacitly condoning systems of oppression.
3) it’s fundamentally untrue. Hatred of oppression doesn’t lead to more hatred; it leads to progress.
4) it is used to attack any attempt by oppressed people to obtain liberation. Point out that something or someone is repulsively racist and all of a sudden you’re “breeding more hate”.
It’s a fundamental misrepresentation of reality that blames victims and excuses fucked up behavior.
Anonymous: why is "hate breeds more hate" a bad thing to say?