Because the government can't have any benefits in cryptocurrency. They can't put laws or they can't even put taxes or any ways to make bitcoin under their control. We know that bitcoin is a decentralized currency which is no one can really control it that's "maybe" why other government want to ban it. Or maybe some illegal activities use bitcoin as a method of payment because it's transactions are anonymous and not tractable.
Actually goverments are trying to tax Bitcoin at this moment, they will let Bitcoin become legal then tracking the buy-sell transaction of Bitcoin through bank's information (Fiat-Bitcoin) for taxing it. Sounds impossible but they always have a way to do it.