Well there are a lot of technology-oriented rich people who want to be able to say to themselves and others that x% of their wealth is in Bitcoin (even if it's

Well there are a lot of technology-oriented rich people who want to be able to say to themselves and others that x% of their wealth is in Bitcoin (even if it's just a hedge bet) and this shared desire to own Bitcoin is reliable enough that it can be quantified; and that shared desire determines the market cap of Bitcoin. It's not different from any other currency. The main difference is that a government cannot use the tax system to force people to use Bitcoin - But then again, the government cannot force rich people to pay tax at all.