I think part of it could be that it would sidetrack meat!rose/rosebot and make her less interested in dirk's scheming if she finds out she has an alternate universe daughter, either vrissy or yiffy. I imagine she would have complex feelings about the whole thing which is not good for scheming.

from what we know about yiffy's backstory she seems to be tough, constantly pissed off, lashes out in whatever way possible at the moment with little regard for consequences (stomping on gamzee's coffin at the funeral), and has zero respect for authority. In a literal sense I think it's that she's a threat to his power fantasy roleplay but I also agree with how she messes with the story dirk is trying to tell for the audience!