BTC is not so easy to pay to use like a credit card for example, it require a few steps, but not so easy for people who are't into technology
BTC is in the hands of the most rich in US, people with high amounts of money and coins, it's not for the poor
It's not a problem. The majority of people use cryptocurrencies either as a speculative asset or a store of value.
The title says "ONLY", but this is subjective. See what I mean, in the US there are over 300 million citizens, 6%=20 million. Not bad I would say, it's even surprising consider all the low information voters there are there.