Regulus was terrified of ending up anywhere but Slytherin. It wasn’t just because he saw how Sirius was treated for getting into wrong House but because he didn’t know anyone else from the other Houses. The other Slytherins he knew, more or less, all his life but knew next to no one from anywhere else. Sirius might have been happy for him but he would have lost far more than he would have gained.
And Sirius couldn’t possibly hold it against him… right?