Registration has nothing to do with it. The lack of AML/KYC controls is the issue. Implementing these would have likely been extremely costly to their business in the form of much lower growth.
Also keep in mind that currently crypto is a lot bigger outside of the US, most of Asia don't want to deal touch things close to US regulators. Eg. it's why Tether is so much bigger than things like USDC (as they actively fight against US regulators in court to protect users).