None of us get to make the decisons about when Holliday, Mayo, Norby or Stowers are promoted to the majors. That decision is made by Elias.
It appears that Elias does not think Mayo is ready defensively to play 3B for the O's. And DH/1B is clogged with players who can hit.
I don't think Elias thinks Norby is going to be better at 2B than Westburg or Holliday. That is why Elias is trying to get playing time for Norby at corner outfield in AAA. Norby has started only 16 games at the outfield corners this season. And only started 54 games in the outfield corners in his major league career. Apparently Elias does not think he is ready to beat out Santander, Cowser or even Hays at LF or RF. One of the reason McKenna is on the roster is that he can backup CF. I don't think Norby has even played CF. So Norby continues to develop at AAA no matter how much he hits.
Holliday appears to be the O's 2nd baseman of the future. He has a few things to work on in AAA in the near term.
Stowers is stuck behind Santander, Cowser and Hays. If he has to beat one of them out he probably does not get promoted. His best shot is if someone get injured.
Kjerstad got promoted but is not seeing much playing time in the majors. Probably has a lot to do with he players ahead of him. That plus his defense, rookie adjustment period and a gut of lefthanded hitting ahead of him.
So we can wish and piss and moan all we want but its Elias that will make the decisions.