These are:
1. Volatility of the Market.
2. They don't have the knowledge about crypto.
3. Negative news that they see or hear in social media/TV Media.
4. They were scammed and afraid to invest again.
5. Threats of hackers in the internet.
But users can also protect their computer from hackers by using:
1. Virtual Private Network for Internet Protocol
2. MetaMask Wallet
3. Not saving any private keys in your PC.
Volatility of the crypto is very high indeed, you can see in the beginning of this year, crypto price was falling greatly from it's peak.
But in my opinion, there are many peoples afraid to using crypto because they still don't believe crypto, thinking crypto just as a bubble or a ponzi that bring benefit only for some early holders.