From a commercial perspective, they're not reef-safe (so you have a limited pool of prospective buyers for them) and they have a long pelagic stage as larvae (~64-113 days for Valentini Puffers; so it's a long time from spawn to being able to sell them, which means it would cost the breeders a lot more time and money than other, faster growing species). Basically, there are just other species that make a lot more sense for commercial breeders to work with at the moment (i.e. there's not much incentive that I can see for professionals to try breeding these right now).
From a hobbyist perspective, they're probably difficult to culture, and very few people would probably try keeping more than one at a time in the first place. I've heard someone reported successfully breeding and rearing Canthigaster rostrata in captivity before, but the link to the report is bad now, so I can't verify that.
