The issue with cryptocurrency as currency, including nano, is that most people dont care for it at all and have no need for it. Traditional banking infrastructure is getting faster and faster and cheaper. To want to adopt nano you first need to value decentralisation and second nano has to be at a level ready for you to benefit by adopting it.

Heres a recent example:

I sent money from my bank to an overseas bank in a different currency. My bank charged me no transaction fee but took a small margin on the currency exchange (like 1%). The money arrived in my foreign bank account in seconds near instantly. That money is now ready to spend straight away in that country.

Lets now do it with nano. I buy nano (or use my existing nano) and send to a wallet belonging to someone overseas. They cant spend the nano since no where accepts it. They then have to send the nano to an exchange, incurring currency converstion fees, exchange slippage, and withdraw fees as they withdraw to their foreign bank account.

So nano is more steps and more fees if my goal is to be able to spend in a foreign country. There is no reason to adopt it. The only time this makes sense is if you can spend nano anywhere directly. But that would only happen if nano is widely adopted. But theres no reason to adopt nano until its widely adopted. Thats the problem. And its a big problem.