Because I think the Strettons would let me keep him if I told him he was a gift from Harry. I just need to make sure Harry will back me up, which means

well I don't THINK he'll care that I'm keeping a rat from Carrow's experiment. He doesn't like Carrow any more than anyone else does. But I'd have to tell him now, I think, so that he could back up my story if he had to. Or at least before I went to the Strettons for the summer.