It's just because Coinbase is being extremely/overly cautious about detecting and fighting fraud. They will refuse to let you buy/sell if your account/documenta

It's just because Coinbase is being extremely/overly cautious about detecting and fighting fraud. They will refuse to let you buy/sell if your account/documentation is a bit suspicious. And they can't explain you why you were tagged (or else fraudsters would simply ask why and would adjust their operations accordingly.)