X3 lol...that and most people don't like to keep 1 species of rays in their tank (at least I think)...and then one day they find out one of their rays is pregnant and then pops out some hybrids lol
IMHO, the other reasons are..... Rays are relatively easy to breed, and even hobbyists with sizable collection can do it..... But for hobbyist, not everyone just keep 1 species in quantity. Usually a mixed bag....