Franco’s Spain wasn’t really fascist, just your typical authoritarian regime supported by a broad right-wing coalition consisting of monarchists, reactionaries, conservative catholics as well as fascist elements. Then there’s the fact that Franco was (de facto) in the western camp after WW2, which mostly consisted of liberal democracies. I don’t think Spain (or Portugal, for that matter) would have experienced democratic revolutions in a Europe dominated by the Nazis and other fascist regimes.
That's convincing too, but then why did the USSR relatively peacefully melt?
I think there should be an alternative, yes, but the alternative doesn't necessarily have to be the predominant one in your camp. The grass is always greener on the other side of the fence, etc. The Spaniards and the Portuguese had more direct exposure to the democracies' customs, and most importantly to their wealth. The Russians' exposure was more limited and delayed, yet I think that when they were seduced by the novelties, they did believe they'd fare better, and soon.
Note, to continue the parallel, that while the non-Russian ethnicities pushed for independence, the key factor was that the Russians themselves wanted a change. In a Nazi-dominated Europe, it matters little if the Danes, or the Spaniards, let alone the Ukrainans, want change; but if the Germans want it...