As for me, there are a dozen reasons:
1. The essence of cryptocurrency is still too complicated for understanding
2. People do not like to study / work
3. Some people are too successful to be open to new
4. The belief that only the state can issue currency
5. Lack of motivation in most people
6. Lack of curiosity
7. Huge Volatility and Big Risks
8. "This is fraud!"
9. "I'm already too old for this"
10. I could not think of 10 reasons. But I wrote a line to make all look like 10 reasons.