A very interesting question. I would appreciate hearing your conclusions and those of the other members here.
Sorry about the delay in replying Tatiana Maria, here is my hypothesis in a nutshell*,
Spain in the 1970's had a higher level of economic development and urbanization than Greece. The Spanish left was for all intents and purposes, exterminated, by Franco's forces during the civil war and its aftermath. Most opposition to the central regime in Madrid was either based in the non-Castilian regions (Catalonia, the Basque country), or came from the right. Spain was not facing any major treats on its borders and its strategic value was limited. Despite a 40 year dictatorship, Spain's political institutions (eg the Cortes) were old and with a few interruptions, continuous.
Greece in the late 1960's however, was still predominantly rural, industrialization and the shift to the cities (in 1967 at least 25% of the population lived in Athens alone) had only really begun in the 1950's. Greece also had greater
perceived threats to its territorial integrity both within (the Slavic minorities) and on the outside (its then communist Balkan neighbors), along with a tradition of military intervention in civilian politics to protect its own privileged status in society, one of the overlooked factors in the 1967 coup was proposed cuts to military spending. Politics was based on clan ties and patronage - the only party that had any real ideology was the communist party. In other words, Greece's difficulties was in a different magnitude than Spain and on top of that was the fact that outside interference was a bigger factor in Greek politics than ever was the case. First Britain, than the USA, had major influence over governments of the day, at one point any potential prime ministerial appointments had to be approved by the US embassy in Athens.
Juan Carlos, compared to Constantine, also became king at 38, whereas Constantine was only 23 when he succeeded his father. 15 years is a huge gap in life experience and the difference in actions during their respective attempts to 'troubleshoot' problems shows. A lot of people often criticize Constantine when comparing his and JC"s relative experiences, but overlook the gap in age between the two
at the time of ascension. Linked to this was the fact that JC had time as heir to reach out to political figures who shared his views and desires (a crown princes party if you will) most important of all, Adolfo Suarez. Constantine did not get this luxury and was unable in the time-frame he had to establish a political identity separate to that of his parents, something was to be a major disadvantage in attempting to reach out to the non-communist left and Greece's liberals during the junta years.
Fundamentally, the reason why JC was able to enjoy his tenure as king (at least until he had to abdicate due to the unfortunate business with the elephant and the puffy lipped lady) was due to Spain being more economically developed and being possessed of a more mature political culture than Greece.
*These are just the main points, there's a lot I've compressed for space and had to leave out.
