Not only asia there are some countries in Europe also where bitcoin is illegal.there have some reason why they are banned bitcoin.such that bitcoin is not mone its a electronic data.so that people didn't pay tax for it and without tax any country gone ruined.so i think thats the reason why some of countries in asia bitcoin is illegal.
Some Asia's country just like Taiwan, Thailand and China have same reasons and that is that bitcoin has no value at all and that it was just manipulating us and some says that it should be not tolerated. They have different different perceptions regarding this matter.
But, in those Asia Japan is really having the great decision by allowing bitcoin and cryptocurrencies to used in making payments and other related transactions.