I think because stocks comes first before cryptocurrencies. With that, more people knew stocks than crpyto. Since we can also earn from stock, they already have spend a lot of time in studying on how to invest and earn from it. They become committed to stocks, they may have no time for crypto, they may see the higher risk from crypto and since they are already earning from stocks, there is no reason for them to find something new to earn.
This is a huge factor, my dad would never invest in crypto currencies but he is trading stocks. And he keeps buying high risk stocks at the wrong time, with cryptos he would have made some decent money instead of losing it with stocks. Another big factor for stocks is that most retirement funds invest heavily in stocks. In my insurance policy I can choose between 15-20 different investment funds. 70% of those funds are stock funds, with a few bond and commodity funds. There is no option do by crypto funds.