The demand for heroin and cocaine is driven by intrinsic value: it gets you high. You can restrict supply but you can't stop people from wanting to get high. The demand will be there no matter what. The utility of the product is unchanged by laws or public perception.
Bitcoin doesn't have intrinsic value. It has value because people will buy it from you. I think BTC is a crappy technology, but I'm not ashamed to admit I'd buy one at $50,000/ea this instant, only because I know I can turn around and resell it for $57,000/ea. The only reason I want 1 BTC is that I know enough other people want 1 BTC badly enough, that I can sell it to them at a good price.
If exchanging my Bitcoin back into fiat or directly for goods becomes illegal, now I've got difficulty and risk involved in using it. I won't want Bitcoins as much. Its utility -- being useful for exchange -- can be directly impacted by the law.
A lot of the people who want to buy bitcoin today would not buy it if it were illegal to do so. I'm talking about casual retail speculators, Tesla, Microstrategy... all of your basic white collar investors who are looking to make a buck but don't want to get in trouble for it. If you delete all those people from the buying side of the order books, and put those who have already bought and want to get out on the selling side, what happens to the price?