Because, when you call trans people “groomers,” you are doing so with no evidence, and you are claiming that all trans people are abusing children without exception. Whereas, when white people are caught abusing children in this way, nobody declares that all white people, without exception, are grooming kids. And the people who claim to be all about protecting the kids from the trans people (who, again, don’t seem to be abusing children in the fashion they are accuse of) don’t seem to care as much when the abuser isn’t trans.
You get upset about abuse of children when it’s claimed to be done by trans people without evidence, but when children are actually abused by people who aren’t trans, you don’t care. So it can’t be about protecting the children, or you’d care about those other cases too. That means it must be transphobia.