Because you simply can't have the same variety you get in 1/35th scale, owing to the size of molds increasing investment, manufacturing, and storage costs. These drive up purchase prices and modellers have to be more discerning about which models they want on their shelves and as a result the manufacturers need to be very selective in putting out models that they know will sell in decent quantities.

Even with the untapped potential of modern tanks, it took Heng Long first the ZTZ-99 (which was probably a domestic-only success) and the global success of the Leopard 2A6 (because it was a much cheaper alternative than the Tamiya offering) to get enough confidence to bring the Abrams and Challenger onto the market.

Thus, with any of the less-known AFV, you'll need to scratchbuild it, get resin kits, or construct them from styrene sheets.