Perhaps because they seem to multiply so well? If you only have to purchase one to have a bunch, then it isn't cost effective for nurseries to grow/maintain/ sell them at a lower price. I looked at a couple sites and every plant seemed to be a hybrid or cultivar which takes time to grow and proof for sale.
Beyond that, I have no idea. My crinums are all pass-a-long plants, much like most of the bulbs and daylilies in my garden.
