Because of how differently they operate, electrostatics need specialized driver units. They can either be cheap transformer boxes that need speaker power amps to feed them, or direct-drive amplifiers that tend to cost just as much, if not more, than the headphones themselves. I think that's the main barrier to entry for electrostatics; at least with dynamics and orthos, you can just plug them into anything and at least get some sound, even if underdriven.
However, it's not impossible to get Stax setups at a relatively low price, otherwise I wouldn't have two Lambda setups by now. You just have to keep your eyes peeled for the right deals.