Because those countries were sanctioned from the very start and the US has no interest in “warming up.” The US warmed up to China because they were both anti-USSR, and eventually that culminated to Deng (and I think Jiang) opening China’s markets to foreign capital.

North Korea and Cuba make allies wherever they can, and those allies happen to all be ones the US hate. They don’t have any leverage compared to a giant country like China being against the US’ cold war rival. Cuba has recent liberalization policies, but it will never matter to the US because it broke the simple rule: Cuba overthrew the US’ order in its ‘backyard.’ They will never be forgiven for it.

Vietnam was basically persona non grata after winning the war despite most of the world supporting it because the US’ sanctions would affect everyone who dared to do business with them. As a result, they were forced to open up their markets as well which is why they were able to develop more compared to Cuba and North Korea. And now Vietnam is in a similar situation as the Sino-Soviet split because it’s a large socialist country that has tension with another larger socialist country, and the US wants to exploit that but Vietnam is taking s cautious approach compared to China who went all in on anti-sovietism

Despite all this, Cuba and NK have still made developments. Cuba with its advanced medical and scientific research and NK with its missiles. In fact Cuba has the same or even higher life expectancy compared to Americans.