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.
I don't think it has anything to do with market share, the iOS betas attract more users because they offer more attractive new features. For better or worse, iOS 7 was a huge redesign and many people wanted their phone to look "cool" early.
Trying to explain to non-techies why Mavericks is better is much harder, aside from better battery life.
For me, the standout feature in Mavericks is the "Ram Doubler" feature (for the old fogies among us who even remember what Ram Doubler was) -- lets you get away with having less RAM.
Given that Apple now solders the RAM and charges relatively high prices for it, the memory compression is a pretty nice feature to have.
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.
