There are a myriad of reasons for the embargo on Cuba. Let's not sugar coat this fact just to use it as an example of "American imperialism".
The reason they drive classic cars has nothing to do with the embargo:
After Fidel Castro assumed power in 1959, he imposed a new law that prevented anyone without government permission from importing foreign automobiles. That turned Cuba into a car museum in the making, sealing the island off from the automotive future.
You should also keep in mind it was possible to get a new car imported, but you needed government permission to do so. This meant only the rich, celebrities and professional athletes were allowed to do so, keeping the majority of the population fending for themselves to keep their old decaying cars alive and working.