Well, those other guys were being called up to cover positions people were (mostly) confident they could cover; Schwarber being the big exception. With Contreras, they still really want/need him to work on his defense behind the plate. Also as good as Contreras is/can be I think it's fair to say he isn't on the baseball savant level that Bryant, Russell and Schwarber were as prospects. There's a lot more factors that go into being ML ready as a catcher, as well. It's not just thinking his bat is ready to throw him out there. Oh, definitely; Contreras is a guy you hope works out and isn't likely the kind of freak you just plug in and he's kicking ass like Schwarber or Bryant (I think it's a mistake to include Russell with them along those lines; he's much more of a traditional good prospect).