Because its a new tech with a new implication. They make their law to protect their currency or anything that may affect them. But i believe once big country already use it, and the application of btc is found in everyday's necessity they will open up and adopt it too. Maybe not all of them but some for sure.
Some governments in Africa are very conservative. In south america too. They don't let people to have their economic freedom by using crypto currencies and they ban it. Simply.
The populations in Africa are precisely the people that should be using Bitcoin because there is rampant government corruption in African countries. Bitcoin is a great way for such populations to keep their money. And the African continent has huge phone usage rates that are rising quickly. According to estimates, half of the population in the African continent are signed up with a phone service, so they could easily start using Bitcoin. People don't know much about Bitcoin there though, and that's why they're not using it.
Some countries in South America need to be using Bitcoin too; For example, Venezuela.
Even in countries where Bitcoin is banned people can find ways to use it if they want to.