Probably because every powerful economy of the past 6 centuries got there due to strong trade, which the US has gone out of its way to prevent Cuba from doing.

No country with a strong economy got there by being forcibly isolated from the world.

And yet the same people say that protectionism is good for the economy.

But protectionism is often good and necessary for a country's economy. If country A cannot produce a good x, then another country B can stifle A's technology growth for producing x by selling x outright at a cost A can not produce the good at (yet). If you ban the import/restrict of x from country B, you will have to pay some fixed costs to get the production of x up to speed; but you will end up with cheaper unit costs in the end. The catch is, you can not ban all imports outright because importing the technology to produce x will probably save you years on R&D and related costs. Like everything in economics, doing things are good and bad at the same time.

Protectionism takes many form and is controlled by the country doing it.

For instance subsidies for US EV véhicules in 2023 is protectionism and will probably benefit US company.

The US do that a lot for a free market proponent.

As opposed to sanction where it’s something that is imposed to you.

You mean sanctions?

That's what sanctions are. Protecting another country from your economy.

Cuba can’t control those sanctions. Protectionism imply that it’s a choice to bootstrap a sector.

For instance : Biden subsidies on EV véhicules apply to US companies only. That’s protectionism.

EU subsidies for EV works with any brands. So the subsidies money are currently going to Chinese EV manufacturers.