Let's get a few things straight about what Glock has done, and what they could do.
To import the 26, they ship them with click adjustable sights that are removed in the states. Without, they fall 5 points short. So the key there is, if they want to import, they ship it in compliance and adjust the gun after it arrives to meet their marketing spec desires.
Now, we look at the 25 and 28. Those guns also fall short on the import scale. A simple sight swap isn't enough to bump over the edge. But....its not how the gun is sold, rather how it's imported that counts on the points. As that form points out, weight with an empty mag goes towards your score. A mag base plate weighing an additional 10-20oz would get you past any point deficit you reach. Once in the US, swap to regular base plates and non click adjustable sights, ship those parts back, reinstall, and send them again.
Now that's weight that needs shipped which isn't free, and it needs shipped back as well (assuming that's cheaper than scrapping). But it would get you past the import laws no different than a Glock 26 coming into the states with a click adjustable rear sight that never makes it to market.
Point is, Glock didn't want to import the 25/28 for their own reasons, as clearly they find ways around the points system as evidence by their 26 not meeting the 75 point criteria as it is sold on the shelf today. My guess is that the 25/28 is a mediocre pistol. They aren't smaller than a standard 19/26. They aren't lighter than a 19/26. They don't hold more ammo. They aren't more powerful. The straight blowback is more snappy and unpleasant to shoot than the more powerful 9mm in a similar sized gun. So there's no real market. Those guns (the 25/28) are made to fit markets where other options aren't allowed but many gun buyers want a pistol. Here, there simply isn't a demand for a 19/26 sized blowback .380.
I'm guessing Glock found it cheaper to open up shop here to get into the pocket carry .380 market than to add weight to ship the gun over the ocean. Maybe a few machines came up in good shape and gave the timing a go. Maybe they simply thought the .380 craze had proven to be more than a simple fad. Who knows. What we do know is they found there to be enough market to proceed with such a gun. I doubt, and I mean seriously doubt there are large government contracts begging for single stack .380s. Even if there were, Glocks model has been to nearly give away the issue guns to then use their LEO popularity as a selling point. I have major doubts that Glock turns much profit off pistols of any type going to those departments, at least in the US. Here, the civilian market is their bread winner.
I can see why Glock wouldn't make a single stack 9mm at this point. When they introduced the 17, it was a radical shift. They created a new market. Time has come and gone and now it's flooded with options, but their reputation has secured them a solid foothold. Today, all that differentiates a top seller from a second or third place product is reputation. The .380 market, from a volume standpoint, has basically been Ruger and Keltec...followed by many small scale runs of different guns. The two big market share pistols both have horrific triggers, that while ok for self defense, are very tough for new shooters. New shooters like the idea of the micro guns for ease of carry first, ignoring shootability. Toss in Glocks reputation and a market still looking for a standout that is both enjoyable to carry and shoot, and it sounds like there is money to be made.
Now look at the single stack 9mm market. You have the Shield, XDS, Nano, bunches of Kahrs, Solo9, PF9, all the compact 1911's, p938 and p290, PPS, LC9, etc it's just a market that's already saturated with all levels of options. Glock has never been one to get into a saturated market.
I'd enjoy a single stack 9mm Glock but I'm not counting on it. They seem set on only breaking into undeveloped or under developed markets, and the single stack small 9mm isn't one of them. Their options cover a wide range of shooting needs. Colt still makes a 1911 and an AR yet we don't give them crap for not reinventing the wheel. We simply thank them for continuing to make quality products as first priority. No reason to look at Glock differently. As long as their options remain reliable and durable, I see them not changing modes drastically for the bulk of their lines.