Probably because most people don't want to have to open their enclosures to get a temp reading. It's easier to set up a probed thermometer that you can glance at without disturbing the snake. That, and the 'higher' price of temp guns, would be my guess as to why more owners don't have/use them.

I do have one and try to use it every time I open the cages to change water or do a general check, tho.