This is not exactly a completely relevant example, but Colorado did see a decrease in teen marijuana use after marijuana was legalized recreationally (granted, it has also recently seen a decrease in teen heroin use, so the legal status may not be directly causal, but I find it interesting nonetheless):
https://www.washingtonpost.com/news/wonk/wp/2017/12/11/follo...
Regarding Bitcoin, one incentive might be positive press coverage - a PR move, so to say, such as when APMEX, the popular online precious metals exchange began accepting Bitcoin in late 2017:
https://www.coindesk.com/major-gold-dealer-apmex-begins-acce...
Many of these cryptocurrencies have non-profit foundations that push for adoption by getting ad-space at major events and pushing for the acceptance of cryptocurrencies in certain companies. Litecoin has the 'Litecoin Foundation' and they managed to get advertising for Litecoin in some UFC events. More often than not, I think partnerships like these make for good PR for both companies involved - here's a piece on why the Litecoin Foundation thought that this particular partnership was useful:
If bitcoins were illegal then I imagine they would probably only be used in illegal transactions.
It's not as though drugs became more popular because they became illegal. Prices may have gone up because they were harder to obtain and there's less competition. Demand was still there because.. well people like doing drugs or are addicted to them.
With Bitcoin, why would a business risk legal action to accept it? There's very little incentive as far as I can tell.