Because not very many people want them. They end up being much heavier than their full carbon counterparts, and sometimes no lighter than their full aluminum counterparts.

Anyway, I know this only qualifies as "semi-deep" but I recently built a set of Kinlin XR-380 rims which are 38mm deep and they seemed pretty nice to me (thinking of building up a par for myself). 550g isn't bad for all alluminum that deep, but it's pretty comparable with the Cosmic Carbon iirc, and most people could get away with 16 spokes on those. The bonus is that they come without hubs and spokes.