The way the game is set up?
QUEST: RECLAIM THE SETTLEMENT
1. Talk to Preston Garvey
2. Go to Graygarden Settlement
3. Kill the Brotherhood of Steel operatives there
4. Talk to Preston Garvey and collect reward
The Minutemen's biggest advantage over the Brotherhood of Steel is a limited one and the Howitzers are only going to be useful in two different cases:
1. When there's a stationary target which they have the position of. Which means that they can target the Pryden or a locaton they've marked the position of. If the Prydwen was moving, for example, they would have been pretty screwed for example but Elder Maxson kind of ****s himself over by just leaving it in the air. They would have been really-really hard pressed if that thing was moving around the Commonwealth.
2. Getting an operative to mark a target's location with smoke signals. Even so, the Howitzers are extremely useful for wartime usage and could be weapons which could make the Minutemen reclaim the Commonwealth from Super Mutants as well as strike Brotherhood positions like Listening Posts as well as the Cambridge Police Station.
I'm not blind that the Minutemen are severley outgunned mind you, I'm just pointing out the Brotherhood retreating after the failed retaliation isn't unbelievable given the catastrophic losses they suffered. Also, I'm not sure how many Brothers are supposed to have come to the Commonwealth versus being left behind to secure the Capital Wasteland and other conquered territories. Really, the Minutemen are the severe underdogs in this fight and their primary advantages are going to be things which will not be useful more than once.
One thing I am curious about, though, is whether we're meant to consider Fusion Cores to be canon or not. Are the Brotherhood of Steel's Power Armor meant to only operate when they have, essentially, batteries they can replace (recharge?). It would make sense but this is the first time the subject has ever been entered into lore and it's kind of ridiculous we have nuclear powered everything and computers drawing power from still-running nuclear plants and electrical wiring but the BOS needs to keep their power armor charged with these.
On the other hand, it's not IMPLAUSIBLE.
Taking a moment to divert from my Minutemen defense, I will say while I think the BoS ignoring the Howitzers is plausible, there's other stuff which the game allows you to do but which I think the BoS wouldn't. For example, you could theoretically stock up your companions with their own Power Armor and BoS-style weapons but I can't imagine the BoS would allow that to happen.
A real set of questions which comes to mind is:
1. How smart is Elder Maxson supposed to be? Is he actually the ubermensch genius which propaganda portrays him as being or is he a puppet for the Western Brotherhood of Steel? He's twenty-years-old and it's very possible he's not quite the Alexander the Great which he appears to be.
2. Is the Survivor allied with the BoS until he turns against them? If so, do they suspect him until he does? How devious is our "hero" in canon?
3. How much damage has the BoS been taking during their war to secure the Commonwealth? How stretched out are their resources by the time of the Prydwen attack? They're currently engaged in wars against both the Institute as well as the Super Mutants of the region. They're also killing Raiders and the Raiders have Power Armor themselves this time but, mostly, I would assume that wouldn't be war but pest control.
Also, potentially the Railroad might be undermining their position.
All of these things would have a serious effect on their ability to wage a war but, of course, don't effect the narrative.
It's also questionable what the "canon" ending will be.