[CadetTommy]

I've always just assumed that it was because IH didn't think they would sell. They probably figured that if someone was putting the money down for a 14 or 16 horsepower tractor, they would also want it kitted out with hydrostatic transmission, which to the average lawn mowing and maybe snow plowing customer was a great feature.
I think it's also worth pointing out that in the first series that broke past 12hp, the 6/7 narrow frames, the highest horsepower manual transmission Cub Cadet, the 126, sold poorly. There were more 73s sold, the budget model in that series, and at that point the 7hp models had already well established their position as the slowest sellers. It would have been reasonable to assume that there wasn't demand for higher horsepower manual models at that time. Of course, IH would go up to a 16 horsepower twin just a few years later with the 582, and then MTD/CCC happily put an 18 horsepower Kohler Magnum in front of a 3 speed and creeper in the 1806.