Because they can't stop bitcoin because it is decentralized and not controlled by anyone even the creator of bitcoin is still unknown so all they can do is stop the exchanges in their countries but still we can use the exchanges by using TOR.
Thats right, the government actually did that already, but they failed. they try to stop bitcoin by giving a ban rules on the use of bitcoin. although it does give a bad effect on bitcoin market but look at bitcoin right now, bitcoin not dead because of it? bitcoin is still alive and still can be used..
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