Nearly all nations in the world already "use Bitcoin" in the sense that they are capable of doing so for any transaction that they want to do and the currency that they use is agreed between the two parties doing the transaction. It'll never become legal tender because of the nature of government regulation, and it'll never replace legal tender for the same reason.
In poorly developed countries, very little bitcoin is used and maybe not all have heard of it and do not know what to do with it or are simply afraid to try it.
Those poor countries are not using bitcoin because they didn't aware of this opportunity and they just need to be aware from this currency and if they get well educated in bitcoin then they can easily use bitcoin and they will know what to do and how to use it as an another currency in their country that can help them especially in terms of remittances.