I wonder if it's because there's little benefit (the ~46 MP sensor Sony makes for Nikon, and the Sony for Sony ~43 MP sensor, are already relatively close to that density). Of course, the new 26 MP sensor is even closer in resolution to the various iterations of the 24 MP sensor (the MF versions are in entirely new resolution territory).
My best guess is that Sony was adding BSI (back-side illumination) to the 24 MP sensor (the 26 MP is the only "24 MP class" APS-C sensor that is BSI), and, for whatever manufacturing reason, shrank the pixel pitch slightly (leading to a small increase in resolution). The 24 MP sensor shows up in quite a few cameras more or less "off the shelf", and in a lot of Fujis with an unusual color filter, and this looks like Sony moving that market to a new BSI version of that sensor that happens to have a bit more resolution (which was almost certainly not the point - the refreshed design was). As we see new "24 MP" APS-C cameras from Sony, Nikon, Fuji and smaller players, I'm guessing they'll all be 26 MP now.
The low-volume MF sensors are almost certainly derivatives of the smaller sensor (the thought process goes something like "time for a new MF sensor - what's our best current layout, we'll just do a big one"). Since the ~43 MP and ~46 MP sensors were already BSI and otherwise quite current, there was no reason to do a new version of those sensors. It wouldn't be hard to do a 24x36mm version at all, but there was no logical camera to put it in - the only Sony-sensored high-MP camera of thew last few months is the Z7, and that more or less shares a sensor with the D850, probably by design. The high pixel count full-frame sensors are mostly somewhat custom (they share a basic design, but they aren't right out of the catalog the way the APS-C sensors or even the 24 MP full-frame ones are - the 36 MP generation were, but the 40+ MP BSI sensors aren't). Nikon probably didn't want to redesign the Nikon-designed parts of the sensor and imaging pipeline (they like to have a bunch of cameras with closely related sensors, so the Z7/D850 combo is appealing to them) - they were already plenty busy with the body and lenses! Sony was just as happy not to have Nikon releasing something with a resolution advantage that sounds bigger than it is.
The A7rIII and D850 are both quite new, with fresh BSI sensors of essentially the same technology as the new 3.76 micron sensors on a slightly larger pixel pitch. I'd strongly suspect that when either gets updated or replaced (or when we see an A9r or D6x (or a D5x, but if there was to be one, it should have appeared), the obvious 61 MP sensor will turn up (assuming there isn't a newer variant of Sony sensor tech before the emergence of such a camera). My first guess would be an A7r mkIV, since that appears to be on a faster cycle than the D850. Sony could, oddly, even sell the 61 MP sensor to a smaller maker before ever using it themselves or getting it to Nikon. It's such an easy design that, if Pentax or someone wanted it, it's worth Sony's while to do a short run of them (it's surely more sensors than Fuji/Hasselblad demand for the 100 MP, and a couple orders of magnitude more than Phase One will buy of the 150 MP sensor).