Firstly, it is the fear in the regime in Havana that too liberal a lifting of trade restrictions will increase the influence of very wealthy Cuban Americans from a circle in and around Miami.
Equally, it is the resentment of those very people at the political nature of the Castro governments since the revolution more than 55 years go.
It is one thing to do a humane deal on the release of political prisoners on either side; it is another for either side to make concessions that might provoke upheaval of the systems they have come to see as their own.
Elements of a mixed economy, however, already exist on the Caribbean island. Private hotel chains from throughout the world (apart from the US) help bolster Cuba’s tourism industry.
The education and health systems are very good.
This sits alongside a steady deterioration in the quality of buildings, serious restrictions on freedom of movement, an antediluvian transport system, and an absence of any political opposition.
When I visited Cuba in the mid-Noughties it was still illegal to own a house. That did not stop an informal, but corrupt, property market from thriving.
Only some of these problems can be put down to the trade embargo. The rapprochement we have seen this week goes some way to civilising a decades-long conflict.
But it would be idle to suppose it means the end of hostilities in whatever form. It will take decades rather than years for the splits to heal.