One, .50 cal is ballistically inferior to the well-established .68 cal.
And two, pistols in general are a niche market- which means they're a niche market of a niche market.
Yes, there's some interest, but that interest is largely already filled by two decent .68 cal pistols. And a .68 can compete on any regular field against conventional markers, whereas a .50 would generally only be good for CQB type scenarios.
So a .50 pistol would be a small subset (CQB players) of a small subset (milsim-ers) of a small subset (paintball players in general) which limits the market somewhat.
A high quality Glock or 1911 copy would also be a $250 to $400 pistol- which would only find buyers of an even smaller subset.
It's simply market forces. Any company today, especially in this still-depressed economy, is going to do their market research, and most likely won't bother tooling up for such a gun, unless they're pretty sure they could sell tens of thousands. And sorry, but in my experience, there's little demand for pistols in general, and even less for anything in a nonstandard caliber.