Why would they allow Bitcoin to continue like this and grow momentum? All they would have to do is outlaw it and 90% of the world population would bitch out immediately and stop. But they don't do it.
central banks aren't a united front. you have to consider geopolitics, and the fact that nation-states are competing against one another.
if one superpower prohibits BTC, they might put their industry and population at great disadvantage to the rest of the world. this is a nascent industry where there is potentially massive economic growth and transfer of wealth. it would be outright foolish to purposely get left behind.
if all the superpowers and most of the others all agreed to ban BTC, maybe you'd be onto something. but that would never happen!