I think the main reason the big millionaires aren't buying bitcoins:
- Not what they usually buy. They listen to people like Buffet and so they invest in stocks and bonds. Products out of their comfort zone aren't even considered.
- They listen to financial advisors and no advisor is going to be recommending bitcoin today.
- They don't know nearly enough about it to even consider buying it.
- They think it's some kind of geek fad that will never get big.
- Even if they are some what interested they have no idea how to buy it, at least not in the quantities that would mater for them.
I think the last point is the most important, especially for average people but also for rich investors.
The difficulty of acquiring bitcoin will probably stay roughly proportional to its desirability. If everyone wants bitcoin, the reasons they want it are exactly the reasons the government and banks don't want you to have it.
At bitcoin's inception, there were already barriers to entry that any competing currency would face. But now we are seeing bitcoin-specific (or at least, cryptocurrency-specific) barriers. If you ask me, that's a dead giveaway that you should suck it up and do what it takes to get past those barriers. Whoever gets over those barriers first will be rewarded the most.
This is also the reason that these nutcase theories that bitcoin was invented by government to track people, is just that, a nutcase conspiracy theory. The government is cracking down on exchanges and arresting people like Charlie Shrem, that is not the behavior of someone who wants it to be widely adopted. They want to limit it. They don't want you to have it because it hurts them and benefits you.