Ive got a theory on this.
What if Gailey looked at the QB's and simply realized Brown needs the most development and that he doesn't want to wait that long.
I mean Gailey has a limited amount of time to build a winner here. He probably knows that they could easily finish the year with a poor record. That means a high draft pick in next years draft. Theoretically, you have a better shot at a franchise QB, the higher you pick. And recent history has shown you can win early with a high round QB depending on the situation.
So, at that point he has to decide whether he wants to invest the time and effort into a guy who could take years to develop knowing they are likely to be picking high and that they may be drafting a developmental QB anyway. He only has 3 spots for QB's on gameday.
And, this year, he's got a vet backup in Fitz that he wants to keep, a starter who is showing signs of life in Trent. That leaves one last spot to develop a QB. If you want to develop one as quick as possible, and its between Brohm and Brown, Brohm is more developed at this point.
Brown lost out because he has too far to develop, Brohm is closer, and we could be in a spot to draft a franchise type QB in next years draft.