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They do. I've seen APAC specific versions where only the Large + sizes have 29er hoops.
Although a trailbike with 29er hoops and 15omm of travel becomes a little unwieldy in anything smaller than large. So there's a case for lots of travel = 27.5". It might not be a great one though, frame designers can make 29" + tonnes of travel work in medium.
Sold my 26" MTB today - sad times. Got the feeling that had I held on to it any longer the 26" 135 QR platform would be so dead that nobody would pay anything for it, despite its awesome slack and Ti.ness.
The thing that's wrong with 26" is that 29" is definitely faster on an XC bike, particularly the way race courses are prepared. The trouble with 29" is that it's just a bit too big for small people. 27.5" allows manufacturers to rationalise inventory with a one size fits all (medium height people) solution, at the expense of not giving anybody outside the 20th to 80th percentiles the wheels they actually need 🙂