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 .
Other countries banned cryptocurrencies because theu fear that this currency might be used for illegal transactions and purposes. They do not want this currencies to be used against the government.
the main reason they are banned of cryptocurrencies is illegal transaction or money laundry
so they are scare that bitcoin will be great in the future and will be the best of payment in the world.