Their countries , their laws. It really depends on your definition for not supported too . If we speak about banning then as far as I know I don't think there is a country that actually banned bitcoin , at least nothing official and If there is then Its related to the banking system with one way or another . If its about the lack of usability in ATMs , local shops then there are plenty of countries and It takes time as some countries have more priorities .
Bitcoin is not supported in some country cause their government doesn't like the fact bitcoin is decentralized currency. Also bitcoin transactions are anonymous so it's another reason that some country that restricted heavily with bitcoin trading then again bitcoin has random value loss habit which made it more suspicious and sketchy to some peoples as well eventually some country's government put ban on Bitcoin. Such things cause trading ban on certain countries and even they take legal action as cause of violating them.