From my understanding, the IRS only has jurisdiction on fiat, and can only tax you if you trade your crypto to fiat. Coinbase is the easiest way to transfer cryptos to fiat, so they would be the low hanging fruit, and easiest route to tax individuals.
All this is just word of mouth information though. Is there anyone who can confirm or deny this?
Actually, the IRS has jurisdiction to tax crypto for crypto transactions, as a taxable exchange of property (in most situations).