The reason is simple. Many governments are not aimed at improving the welfare of their citizens and therefore prohibit bitcoin. Or they are afraid for their national currencies, that they can not withstand competition with bitcoin.
Sometimes the reason is because the people itself who use bitcoin for illegal purposes. We must not abuse the freedom that it gives us. Bitcoin is helping country for the economic growth and also for the welfare of the people. We must not do something that will give reason for the government to ban. Use bitcoin and investing in good way to earning and live comfortably.