The Americans put pressure on Cuba to not admit Snowden:
"Snowden failed to board an onward flight to Cuba the next day, the paper says, again citing Russian officials, because the US put intense pressure on Cuba, warning of "adverse consequences" if Snowden were allowed to board the plane. The Cubans subsequently informed Moscow that the regular Aeroflot flight would not be permitted to land in Havana if Snowden was aboard."
