It's could be a possible reason is we are still at the early age of crypto, people are still confused about what is crypto. But I am sure that as time goes by crypto is getting a lot of attention from people especially those big financial companies, they are starting to what crypto can do.
And another reason is because of the illegal doings of other people that using cryptocurrency, for example, using it to do some illegal doings, especially for buying illegal drugs. Different hacked/scams involvement of cryptocurrencies, especially in some crypto exchanges.

What I think to be the reasons why crypto will become Mainstream?
-For me, first, it is on Bitcoin, especially the block halving of Bitcoin. If we can be based on the previous, for every upcoming block halving of Bitcoin, the cryptocurrencies market cap are increasing, like a few months or a year before the halving, we can see the crypto starts to become mainstream because the market cap is increasing.