There was a somewhat understandable reason initially; when the embargo was first started, it was because Cuba allowed the UUSR to use it as a forward base for missiles so they could reach the mainland US, which, understandably, the US wasn’t very happy about
But ever since the fall of the USSR it’s been absurd
It’s also worth adding, though, that the convention of only running for at most two terms had existed pretty much since the establishment of the republic (until FDR broke it), when Washington and Jefferson each chose not to run for third terms