As a matter of fact, XBT Provider (Swedish Bitcoin ETN) is widely available to US investors through most brokers, and it does very impressive volume compared to ETFs in the US.
There is a distinction. While both are securities, ETNs are generally unsecured debt notes. ETFs provide shares into a fund that holds the underlying assets. They carry different risks — mainly, credit risk vs. tracking risk (whether the fund actually tracks the underlying):
https://www.investopedia.com/investing/etfs-vs-etns/