Western Union being free is the outlier here.
Which doesn't has anything to do with the fact that if you've got Bitcoins in a village in some random developing country, you need to find somebody that's willing to give you local banknotes. That person is usually going to charge more commission than Western Union for the privilege of doing so, because they can.
The country switched to using BTC as legal tender because it was already being used as local currency because the people were using it for payments due to the lower fees.
> That person is usually going to charge more commission than Western Union for the privilege of doing so, because they can.
It seems like you're assuming this to be the case without having any real knowledge of the situation. BTC was adopted by El Salvadorans exactly because they were able to spend less on fees over the competition.