I am really curious about airports like MSY, I think those are very curious given they not big numbers and also may not have crews based there, so it is interesting. I mean I know it is a giant puzzle and sometimes a plane might stay where it is because that is cheaper/easier. I never would have thought MSY would have been a place UA would put a plan for any length of time. BOS is an easy one as I know it was maintenance base for pmUA and they even had the nice hanger (with the great image on the outside as you would exit Logan, had many a views of that when I lived there).
Again, there are probably a bunch of other people who know better than me, as I am but a humble air traffic controller, but I'll take a crack at it.
Most of the outstation locations were probably chosen based on a few factors. I'm sure cost ranked pretty high up there. The airlines obviously want to park them someplace cheap and practical, and airlines want revenue any way they can get it right now. Location is also a factor too. We're headed into severe weather season, so it wouldn't be ideal to park too many airplanes in a place at risk of high winds, tornados, hail, etc. Maintenance I'm sure it's a factor as well. I know that UA used to have maintenance at PHX (presumably still does), so that combined with an ideal climate and relatively low costs makes it a good location. Having mechanics already on payroll at the outstation make it easy to maintain the aircraft while they're parked. I don't know about MSY, BOS, etc., but I'm willing to bet that UA has some mechanics at most of those outstations. The company can't furlough or lay off mechanics for a while, so they may as well keep them working even though the airplanes aren't.