My understanding is that the political parties pretty much ignored the past in order to "heal the wounds" of the civil war. So PSOE did not run for election with the aim of "avenging" the crimes of Franco and there was no large scale prosecution of the old regime.
This seems odd to me because of the power the left had back in the civil war. Where did all these anarchists and communists go?
When Franco died, there were three given possibilities: keeping a similar regime, under Juan Carlos I; transitioning slowly to democracy, or breaking with the past.
Transition was supported by the biggest parties (UCD, PSOE) and the King and Suarez, who became a strong political leader during the late 70s, strongly symbolized that path. It won.
PCE's (communist party) leader Carrillo (and union leaders) actually had a to make a decision. Go and try to instigate a popular movement against the transition and therefore "to play out of the board" (like revolutionary parties did) or stay in the transition path. He chose the latter.
PCE was the symbol of the left and resistance in Spain. When Carrillo took that path, revolutionary left "couldnt lead the left strongly enough to break with the transition. (maybe they could, but they didnt)
So left lost that opportunity. We (Spain) chose the path of transition. Not so long ago (2013/4) PODEMOS party even talked about "breaking the transition regime"). This is quite important, still today.
EDIT: "revolutionary left" against "Carrillo" is not a personal consideration. Historiography has indeed considered (is that exactly good) a divergence between "carrillism" and parties who actually considered the transition a starting point for a revolution. Thats why "they lost their opportunity" if they had it.