My feeling is because people haven't bred vars of them yet.
Once vars are available, their popularity will increase big time.
Takes someone special with a sense of risk to try to make strains. A lot of thanklessness to it, and almost no glory. But personal reward is there.
I swear I remember reading that it took 7 years to breed the original color of red cherry shrimp. I sure appreciate the person who did it. However his name is all but erased from history. He's basically unknown for all the work he did. IMO He should be immortal in the shrimp world, but instead- everyone was just concerned with his creation.