I suspect boycotts galvanize tyrants. They need an "us versus them" narrative to justify their actions and prosecution of their enemies. It provides a wonderful scapegoat for domestic issues too-- I suspect the Cuban government could write off mediocre economic growth for years as "hey, the biggest most obvious trading partner won't deal with us."

It's a lot harder to get people rallied against an "enemy" who supplies your food/favourite consumer goods.