.43 has been the standard for magfed milsim since well before .50cal was reintroduced. The chinese company that produces 'realistic' blowback pistols for... well, just about anyone who sells them, has got a proven line that includes something like a Sig, a pseudo-Glock and a Desert Eagle lookalike. They've honed that product over generations to be quite functional, and they've got a lockdown on the caliber.
Kingman did make a .50cal pistol for a while, and the failure of .50cal has caused them to switch to .43 instead (granted the whole line is now out of production). If the success of Kingmans product had shown there was a market, perhaps a new company would have tried, but as it is there are plenty of .43cal markers on the market, and that caliber was embraced by players looking for that type of marker almost a decade ago.
.43cal has outlived .50cal in a competitive sense; it's been over a year since GI Milsim closed its .50cal division, GOG and GI Sportz haven't picked up the reigns, and conversion kits were short lived or failed to appear. Sure, KEE has put .50cal into their plastic spring-powered Splatmaster revival, but I don't think anyone would argue that it should really be considered as anything other than a branch off of paintball.
Anyway, it sucks that you won't get a Desert Eagle in the caliber you're looking for, but in a couple years when you can't find .50cal paint outside of blowgun websites (again), you'll be happy to know you can still order slightly overpriced .43cal paint from RAP4 and probably a few other companies that survive or spring up.