There are folks who do enjoy bigger calibers,, heavy recoiling, and capable of presenting more power than really necessary. But the market, is smaller than what a big company can invest in.
Custom gunsmiths, smaller companies can adapt easier,,, and make enough to satisfy the niche market.
As dweis has mentioned,, investing in improvements, advantages, or things that sell to the masses makes more sound business sense for a company that has stockholders to satisfy. Just this past weekend, at my USPSA match,, a guy showed up with a Ruger PCC. He had shot mine,, and liked it. He's been using a CZ PCC for a few years now. This was the first match he'd tried with it,, and said he'd only fired a few test rounds through it before the match. A new gun,,,(to him) and he was VERY happy. I heard him say; "I may well be selling my CZ now!" The fact that his Ruger PCC can use Glock magazines,, was a big selling point to him. Innovation, adaptation, and going after the masses to sell a lot is smart business.
I've shot a few 500's in a few different guns. I'm not recoil shy, and I'd do it again. But would I SPEND money on such a gun? No. I have a few .480 Rugers,, and if I need a big bore handgun, I'll stick with that one. I also have a .454 Casull and it's loads aren't to it's full power potential. (It's a FA,, and the price was very, very reasonable. And I'm a sucker for the .45 cal stuff.)
I think if Ruger were to offer any variation of a .500,, it would sell some,, but overall, it'd be a marketing disappointment.