I don't know as much about this next part, but seeds are also harder to import. I'd hazard a guess at there not being much capacity in that market unless you're buying from Monsanto or the like, which seems not to be an option in Cuba as those (American) companies are not exempt from the embargo, unlike exporters of food and medical supplies.
[*] According to many people, I’m not talking about the parent comment.
A decent intro to Monsanto (now Bayer, really) that covers some elements of their monopolistic behaviour: https://www.ethicalconsumer.org/home-garden/monsanto
tl;dr, there is a monopoly, and america decides if that monopoly can do business with Cuba
As they have fewer baskets to place their eggs, they are much more succetible to crisis: a blow in one economical activity generates crisis that affects a lot all other areas. For example, one of the causes for the current crisis is the pandemics that dropped their tourism to zero for 2 years. Now they have much less to invest, affecting all economical activities.
Which is enough, as the US controls the emission of dollars, have monopolies over several technologies and, as it extends the sanctions even to the ships that trade in Cuba, disallowing them to enter in US, on practice it difficult trades between Cuba and any other country.
> It doesn't explain why Cuba needs to import 70-80% of their food despite being a fertile tropical paradise.
Modern agriculture depends on machines and fertilizers. And if you go without modern technology, climate change and shortages of water can be an even greater blow to your production. You will produce food, but will be susceptible to famine crisis, as were all countries before sufficient technological development.
Moreover, Cuba is also not an agrarian country. The urban population is greater than the rural, like in any modern country. Remember that societies without much technology had to have a much greater part of population in rural areas instead of cities to be self-sufficient in food.