>you can still buy those on margin

That is absolutely not the case in a categorical sense.

Brokers can and do apply different margin requirements on a stock by stock or customer by customer basis. There's nothing out of the ordinary about a security not being marginable or having increased requirements due to risk assessment.

For example, here is a list of stocks at one broker with particular margin requirements, and it says it "changes frequently":

https://invest.ameritrade.com/cgi-bin/apps/u/MarginReq

Given that there are hundreds starting with "A", I'd assume the entire list is in the thousands, of securities that have individual margin requirements.

I think in fact the ticker GBTC which is a trust that owns Bitcoin isn't marginable.

Doesn't the market volume have a multiplicative effect, though? In other words, it's not just volatility, it's volatility x volume. Or something like that.

That's true. Maybe something to do with regulations then? HSBC is already being accused of being involved with money laundering quite a lot. Also, what other coins except BTC have market cap >1T?