Either the Cubs think Mervis still has things to work on in AAA or they simply don't view him as the answer at 1B. If they thought Mervis was going to come up and make an immediate positive impact, I don't think they would be holding him back on account of undeserved loyalty to Hosmer. There is a third possibility that the Cubs view Mervis so highly that he was only being kept down for service time reasons, but they learned from the Kris Bryant situation that it is better to wait beyond the exact service time date to give themselves plausible deniability. I really don't think that's the case for a guy who is already 25 and not a top tier prospect, but you never know.