It is not impossible actually there are some countries that already banned bitcoin and that includes:
• Bangladesh • Iceland
• Bolivia • India
• China • Russia
• Ecuador • Sweden
• Thailand • Vietnam
This is according to the blogspot I've visited a long time ago:
https://www.ccn.com/top-10-countries-bitcoin-banned/But here's the all the list where bitcoin is legal or illegal:
https://en.wikipedia.org/wiki/Legality_of_bitcoin_by_country_or_territoryBut in my idea about why the country banned bitcoin is that it may heavily affect the economy once the bitcoin take control of the economy the effect will be:
1. Bankruptcy (Because of the small amount of people who use the bank.)
2. Blackmarket/Deepweb may become powerful since we are anonymous online.
3. The Government can't put taxes to the people who works in cryptocurrency industry.
4. If the country uses bitcoin as their mode of transaction some business will be in bankrupt.
5. Bitcoin can be used to launder money.
The basis of this reason is just my observation, idea and own opinion.