I suspect it's because iOS devices feel far more disposable than ones main work computer. By that I don't mean in a product design sense, but in a data loss sense. People are used to the idea that their phone might lose their contacts, or some other important data, and therefore keep them well backed up (iTunes even has a utility to do this). With peoples main computer though, the possibility of something going wrong seems far more dangerous, even if the probability is similar. People often back up their main machine far less often, and care more about the data on it.

It's probably because iOS betas attract a lot of non-devs. OS X market share is pretty low compared to iOS.

I might be wrong, but I think I read somewhere that Apple would reject submissions to the app stores that were built on beta systems.

If I'm right, that would explain a lack of uptake on the betas for developers, since that would be a bit of a dealbreaker unless you are juggling multiple macs daily.