Because blockchain-based cryptocurrencies have a terrible UX and don't scale for shit.
Even if someone hand-delivered a smart-phone that had a Monero wallet on it to facilitate me paying for a banned_book_123, it's still an order of magnitude fussier and slower than, say, using a Visa gift card. Or cash.
And what happens if I somehow fuck up the transaction? It has none of the protections offered by the Visa gift card, with all of the shit-out-of-luck properties of physical cash.
On the other hand, it doesn't scale well and thus cannot be used for digital microtransactions.
Any of the solutions I see for scaling could just be used without having a blockchain-based cryptocurrency on the backend.
Anyhow, when it's this difficult to use, it seems perfectly logical that it only gets used in niche cases where the sender/receiver is willing to eat the cost of all the friction, headache, and risk-of-loss of using it (not to mention price fluctuations). So ransomware, paying for illegal goods... what else is there?
Related: I think the motto of blockchain should be: A Solution Begging for a Problem. All these HN posts about it make me think of "The Herlihy Boy" from SNL:
https://www.youtube.com/watch?v=oXyW5w5RRFY