The wikipedia page on the flag of Cuba says:-
It is one of the two flags of a currently socialist country (the other being Laos) that does not use any Communist Symbolism.
The symbolism is (from the same page):-
The three blue stripes represent the three departments in which Cuba was divided at that time, the white purity of ideals, the light; the red triangle, originating from the French Revolution – and the three ideals of liberty, equality and fraternity: red for the blood and the courage; the star was the new state that should be added to the United States.
The flag was adopted in 1902 but the Cuban Revolution happened fifty to sixty years later! Why didn't the new regime do away with the flag or add any communist symbolism (same goes for aforementioned Laos) to it to distinguish it from say, Puerto Rico (the design is the exact same but the colours are flipped)? Some of the symbolism is also quite surprisingly contradictory to communism!
Side note:- The current flag was adopted in 1902 but dropped again in 1906 for a flag that is the same, just with a lighter turquoise-ish shade of blue. The current flag was reinstated after the revolution.