They do not want to invest their money simply because they do not believe in something that they cannot have a hard proof and they cannot have it physically. Cryptos are digital and it is difficult to persuade some maybe because it is not regulated by the government so they feel that there is no security at all.
Another problem is that crypto is not usable in local market till date and will take a long time, while regular currency can be used anywhere. Mostly people use their money in their local markets and crypto cannot be used in local markets. It is also a big problem for small investors that they do not invest their money in cryptocurrency.