Why is Power Armor so much more common in Boston

In Fallout games prior to 4, functional sets of power-armor are a relatively rare commodity (at least, outside of BoS/Enclave installations where power-armor may be common but mostly also non-lootable...especially if you don't already have some of your own). You generally don't get your first set until mid to late-game, and outfitting an entire party with power-armor takes a lot of effort.

Contrast this with Fallout 4, where...well:

I've got 7 sets of power armor, and I haven't even been to Diamond City yet. The first set I found basically by accident (went up to the rooftop before the guy told me to), and more sets keep turning up more-or-less the same way.

Is there any canonical/in-game explanation for why liberated/lootable power-armor is almost commonplace in Boston and so rare everywhere else?