I believe it has to do with a number of factors including: Burms, I believe, have been available as pets longer than Retics, the clutch size of a Burm, and the number of people that can realistically take care of a Burm. There are a lot of Burms being put up for adoption and being let go to fend for themselves. I don't hear about that nearly as much for Retics, so I imagine the supply/demand for a Retic is quite a bit less than a Burm.