They made the 42 locked breach like the rest of the product line, they could do the same with any caliber if they chose to do so.
The Glock 42 is a completely different gun designed especially for the .380ACP cartridge and especially designed to be locked breech. It's also much smaller and lighter than either the 25 or the 28.
If they made the 25 and 28 into locked breech guns they would have to change things considerably to get the gun to operate reliably. They didn't go to blowback for the heck of it, it's because the .380ACP won't reliably operate a locked breech system designed around the 9mm unless you modify it.
So they did a very simple modification to the barrel (maybe the slide too, can't remember for sure on that) that changed it into a blowback system. That allowed them to keep nearly the entire gun the same, simplifying manufacturing and keeping costs low.
And if you look at say a 17 and build the same gun except in .380 with locked breach your looking at a gun with higher capacity...
The capacity would be the same if the dimensions of the gun/magazine stay the same. At its largest point, the .380ACP is 0.02" smaller in diameter than the 9mm. Multiply the diameter difference times 17 and you don't get enough extra space to shove another round into the magazine. If you made the magazine slightly larger or changed the magazine design then you could probably add a single round to the capacity. We might have guessed this without the need to resort to all the complex mathematics by looking at the 25 and 28 and noting that they both have exactly the same capacity as their 9mm counterparts.
Also, as noted, you can't just rechamber a full-sized 9mm to .380ACP and expect it to work properly any more than you can rechamber a .380ACP to 9mm and assume it will work ok. Other changes would have to be made to get the gun running reliably. And by other changes, I mean significant changes--i.e. more than just a spring change. Locked breech guns are momentum driven and springs have less effect than most people intuit.
Anyway, the market for large .380ACP pistols is pretty low. There have been some pretty nice mid-size to fairly large .380ACP pistols on the market that have been discontinued, presumably due to lack of market interest.
For a .380 version they buy a new reamer, make a new mag, and weaken the spring and suddenly they increase their potential customer base.
If it were that easy and the market is crying out for full-sized .380ACP pistols, why isn't anyone making conversion barrels and modified magazines? I mean, you can readily get conversion barrels in 9x25 Dillon and that's not exactly a round that's jumping off the shelves.
Clearly there are only two possible options:
1. EVERYONE in the gun accessory/gunsmithing/caliber conversion industry has colluded with Glock to keep full-sized .380ACP pistols away from the American consumer.
OR
2. Absolutely no one but you has figured out that the American gun-buying market is starved for full-sized .380ACP pistols and that it would be super-simple to satisfy that request.
As much as I am into conspiracy theories, I'm thinking that it's option 2. Therefore my advice is for you to start contacting gunsmiths and gun makers and explaining how they're missing out on the chance of a lifetime. Once you give them the technical details on how to do the conversions so they can start raking in the cash I'm sure they'll happily reward you accordingly. Don't let them try to tell you that it's not as simple as you think it is--just let them know their objections are hogwash. They'll come around eventually.