Because they can be used for long-distance transactions that are supposedly anonymous, they are often popular with people attempting to evade government regulations - presumably including US restrictions on sending money to places, such as Cuba.
If that is only the reason why people and Cuba, or any countries that accept and legalized crypto, that is a wrong idea. It is certainly not a good decision. They legalize Bitcoin/cryptocurrencies as they have the insights that their people were getting beneficial with this, not to evade the government or any restrictions otherwise it was a subject for the ban.