why do you think it’s the only factor?

"Material conditions" isn't a single factor, its a plethora of interlocking economic systems.

Meanwhile, there is no singular psychological profile that defines "billionaire". Elon Musk and Warren Buffet and Michael Bloomberg and Miriam Adelson and Donald Trump are all totally different people.

There were thousands of individuals who had the right connections, the right age, the right group of friends, etc.

Quite a few of them become mega-millionaires. It's not as though only having $500M makes you some kind of failure. Some people just get luckier than others. The folks that got rich on AOL and Yahoo never reached the heights of Google or Comcast, but it hardly matters. They're still stupid rich.

I think there is something that sets apart one class of obscenely-wealthy oligarchs from their slightly-less wealthy counterparts.

Sure. Dumb luck.