Generally happens with physical editions; they need to be shipped over the pond. Ebooks are generally available on the official day of release.
With the 2009 Kelvinverse movie, Simon & Schuster was shipping hardcopy Trek novels to Australia so much faster (no more sea freight!), and my local shop in Sydney was selling them as soon as they arrived, often beating everywhere else. Sadly, about two years ago, that slowed down again (different distributor) and now we are several months behind US release dates. Drat.
I noticed that the new TNG novel, Shadows Have Offended, has arrived in a local bookstore. For anyone who is keeping watch for it.
Mass-market paperbacks are no longer marketable; the industry today just doesn't support them, since e-books have largely taken over their niche and book vendors prefer trade paperbacks. This has been the norm with Trek books for the past several years, as well as with the publishing industry as a whole.
Or you just stop publishing mass-market paperbacks since they're not profitable anymore, as opposed to pricier trade paperbacks. Mass-market publishing was always grossly inefficient. Often you'd have to ship three books just to sell one, with the other two copies being stripped and pulped. And the publishers ate the cost, not the retailers. It only made sense when you could hope to sell lots of copies to both the trade and wholesale market. And, as Christopher noted, ebooks are cutting into that market, plus lots of grocery stores and drug stores and convenience stores barely carry paperback novels anymore. And I have my concerns about airports and train and bus stations. Last few times I traveled, pre-pandemic, I couldn't help noticing that paperback readers were a distinct minority. Most travelers were diverting themselves with their devices. I remember being told, back in the nineties, that a mass-market paperback wasn't really worth publishing unless it was expected to sell at least 20,000 copies. That was back before ebooks, though. Not sure what bars you need to clear these days. Let it be noted that I'm NOT speaking for S&S here. I'm not privy to their internal business decisions. I'm just talking about paperback publishing in general.
MMPBs were meant to be disposable. Trade paperbacks are higher-quality products, meant to last longer. Besides, there have been Star Trek trade paperbacks for a long time, though they've usually been anthologies or omnibuses. It's just that instead of having parallel trade and MMPB lines, now there are just trades. (Though I think it would be nice if they started doing e-book exclusive novellas again.)
And they were meant to be convenient to carry around, as in "Pocket Books." Now that people can carry a library on their phone, it's no surprise the market for cheap, portable paperbacks is going away. As I understand it, the modern mass-market paperback evolved from the old Armed Services Editions produced by the U.S. military to entertain bored G.I.'s back in WW II. They were meant to be cheaply-produced, lightweight, and disposable: something a soldier could carry around in his backpack and possibly share with his bunkmates before they fell apart or got left behind at the base. As the story goes, the ASEs got American soldiers into the habit of reading cheap paperbacks -- and enterprising publishers jumped on that in the post-War era. But that was a long time ago. Mass-market paperbacks may be going the way of radio dramas, pulp magazines or VHS tapes. https://en.wikipedia.org/wiki/Armed_Services_Editions
It's not quite that dire - the format's going away, but you can still get the content in a couple of mainstream formats. That doesn't really apply to radio dramas (outside of BBC Radio 4, at least as far as mainstream goes - Big Finish and podcasts don't count) or pulp magazines as far as I know. I wonder how many people clamoring for mass market paperbacks to return would do so if the prices were properly inflation-adjusted and they were $12-13?
It's booksellers and publishers that do most of the throwing out, since sellers are required to return unsold copies to be pulped, since it costs less for the publishers than storing them. It's a very inefficient system. We talked about this in the Trek Lit panel at virtual Shore Leave this past weekend, and it was pointed out that trade paperbacks are "greener" since there's less waste involved.