Because almost no administration was ready to regulate it until recently. Payment and remittances are such obvious use cases but if every transaction create a tax event, it's also too cumbersome to implement for most merchant.
If the government has to regulate cryptocurrency for it to be useful as a currency, then it's not very useful as a decentralized currency, is it? In my view, most cryptocurrencies (bitcoin, at the very least) are built on fundamentally unsound economic principles which incentivize hoarding and speculation.
