My parents thought that bitcoins that cannot be seen or touched cannot be an asset. They think that investing in physical assets such as gold, bank deposits, land etc. is more profitable than Bitcoin. But even after I convinced them, they are not ready to invest in Bitcoin. They don't want to believe in virtual currency.
Maybe your parents have no experience with cryptocurrencies so they want to hold their money in physical assets instead of bitcoins. The older generation is not as educated as compared to our new generation which is why they don't believe in virtual currencies they think they are all scams.
I think one way you can explain to your parents is to tell them that investing money in physical assets is more profitable than investing the same amount in bitcoins. Maybe they'll be tempted and tell you to invest in bitcoins.