> the people most oppressed by Franco
Let's at least agree that this is subjective.
Given there were no other options, and few people wanted to return to dictatorship, the new constitution was adopted. But that doesn't mean it wasn't a shotgun, Hobson's choice between dreadful and merely bad. Why should people continue to adhere to it?
https://www.boe.es/boe/dias/1977/01/05/pdfs/A00170-00171.pdf
Read the first two articles of that law; Spain was no longer a dictatorship. The last clause of the law enacts it as supreme law of the land.
It is following that law that the first Congress of the new democracy (with the mandate to write a Constitution) was elected, using the same procedure we keep using today to elect our Congress. In that election, the francoist party of Fraga lost immensely to the centrist and marxist parties.
If the Constitution they wrote didn't pass the popular vote of the referendum, general elections would be repeated and the process would start again.
It never was a choice between accepting the new Constitution or going back to a dictatorship. It's called "Transición" instead of "Revolución" because the francoist regime turned itself into a democracy following its own laws and procedures.
That's a very optimistic view of the situation. In practice, the instability resulting from not completing the process would have had dramatic consequences. Among other things, there was a Cold War on which could have easily started a new civil war.
> the francoist regime turned itself into a democracy following its own laws and procedures.
But exactly because the prime mover was the regime itself, there was always a chance it could go back on its steps. Compromises had to be made to ensure the transition would go through smoothly, as it's always the case.
> It was drafted by 7 hand-picked representatives, 4 of which were basically franchists
Two out of the seven writers of the Constitution were Catalan (~1/3 of the writers for ~1/6 of the population). One of them a communist. The other one, exiled during the dictatorship for supporting Catalan nationalism.
> ratified by franchist institutions before being put to a popular vote
Before being put to a popular vote, it was ratified by the Congress that resulted from the first free elections in Spain in 40 years. The francoist party of Fraga only won 16/350 seats in that Congress; comparable to the 11 seats obtained by the Catalan party of Pujol. Half the seats had been won by centrist voters, and the other half by marxist voters.
This Congress was our "assembly of people elected democratically", that could tell the writers to go back to their drafts until the result was to their liking. Those seven writers, by the way, weren't "hand-picked": they were picked by that Congress, among their members; they were all freely-elected representatives.
The prime minister was still an ex-Franchist and the UCD had plenty of old hands around. The Franchists still had huge relevance in the army. The drafters were still moving with a somewhat limited room for manoeuver.
> could tell the writers to go back to their drafts until the result was to their liking.
Not really. These things don't happen in a vacuum, the longer the process and the higher the risk that it things could go pear-shaped. There was active terrorism and of course cold war activity. Compromises were made, which were good for the times but not necessarily the best overall.
I'm just stating that no, the writers of the Constitution weren't "hand picked", rather selected by Congress among representatives elected democratically; and no, it wasn't ratified by francoist institutions, rather by a democratically-elected Congress (and later on by a referendum). That's just revisionism that gets spread to fuel Catalan nationalism.
... was drafted 40 years ago by a Franchist establishment concerned with maintaining a certain degree of continuity. It's not surprising that the people most oppressed by Franco are not particularly happy with the result.
Besides, the Wilson doctrine is usually more relevant for this sort of movements than whichever piece of paper is supposed to be law here or there. A population cannot be denied the right to self-determination, which includes splitting off if they really want to. It can be a stupid move (see Brexit), but people must be given the possibility to democratically discuss it and vote on it; because otherwise things will eventually go full Balkan, and you never want to go full Balkan.