There is more than that.
Teams promoting top prospects can now earn an extra draft pick if the player accrues a year of service time per the PPI (Prospect Promotion Incentive) in the CBA.
A prospect has to qualify by being a top 100 prospect on X number of lists and have less than 60 days service time in his career. Mayer certainly meets these requirements. If the Sox promoted Mayer before the service time deadline and let him accrue 172 days on the MLB roster, the Sox would receive an extra draft pick after this first round, in what we used to call the sandwich round.
Jackson Holliday was promoted with 173 days left in the season for this exact reason. It certainly helped that he was absolutely raking in the minors and that some of the Orioles' infielders (notably Ramon Urias) were struggling. Story went down with about the same number of days left, leaving this an opportune time to promote Mayer as well. Except that at the time, Mayer had not yet played a game since August 2nd. This was probably a very large factor in not promoting him. And if it was not a factor, it damn well should have been.
The Orioles might have been able to promote Heston Kjerstad as well, as he was eligible for the PPI. And the early struggles of Santander and Austin Hays certainly justified getting him into the lineup. Not sure why they did not, unless there is a limit to how many PPI picks a team can accrue. Or it could be as simple as they have faith in Hays and Santander ending their slumps, unlike Ramon Urias. They also might have promoted Kjerstad if Ryan O'Hearn was struggling, but he is flat out mashing right now...