The .500 caliber is a decent one, but more powerful than most are willing to put up with or need. A bit on the expensive side and there are ranges that limit or won't let owners shoot them.
Like a lot of different revolver cartridges a majority of the buyers were getting these as novelty items, getting rid of them or considered themselves stuck with them.
Gun magazine writers such as Taffin and Clint Smith have been documenting this phenomenon for decades. This includes recently,too.
This being a special purpose cartridge as opposed to one more versatile don't help things a bit.
That said there is nothing wrong with this cartridge for those who need it or just want to have a mega blaster for fun. That's all part of shooting.
However extremely high pressure revolver cartridges and guns come with a whole pack of criterea that have to be dealt with.
Lower pressured rounds do not or may not do so.
There are also a plethora of big bore high pressure rounds that are in direct competition with the .500. Casull .454, Ruger 480, S&W .460 as well as host of others ( wildcat cartridges included) are all vying for the market dollar. That's a steep mountain to climb. Kind of like trying to go up Mt. Everest when one is used to the Great Smokey Mountains.
For those who enjoy the .500 regularly, sorry to see that happen.
There are many varied reasons that the market can't sustain thngs for those who enjoy the cartridge and the big booming revolvers.