The fundamental philosophy behind the revolutionary movement was nationalist and formulated around the idea that the Batista government (and the entire Republican period) had betrayed the ideals of the first Cuban revolutionary insurrection. Castro's revolution wasn't meant as a break from the past, but as a culmination of a revolution that began in 1868. For that reason, the Cuban flag, which was designed precisely as part of the revolutionary struggle against Spain, continued to be an important symbol for the revolutionary government post-1959. I can explain further, but hopefully that helps?