Because there is not much in the part of TX that they come from, the hobbyists who have them tend to sit on them (not sell them, or in one case - only sell to Japan), and the people who know where they are don't share any of the information.
I've driven down to that part of TX, setup a lights and came back with nothing. Have offered to buy from multiple people and also have asked several people for hints on where to look.. nothing yet (it's been years).
Many more people (including myself) collect south of Tucson at well-known locations that are on public land and are easy to get to - and also in the time of year when all the other inverts are out. M.v., on the other hand, is found in the fall.