Totally agree. Yet, the 70s were different times. Getting out of a dictatorship and moving into a democratic state takes a lot of compromises on all sides. Back then, there was the sense of urgency to agree on common terms, for the time being, to allow the country and the people to transition into a modern democracy.
It's been 40 years now. Times change, the world changes, people change. A constitution is a very important document for a country, yet it should not be used as a license to oppress people.
Many Catalonians requested to change some parts of the constitution to better represent Catalonia's cultural and political differences, as compared to Spain. Yet Spain would not even consider talking about anything that challenges the status quo.
We will see where this is heading.
There are legal ways to change a constitution. In all cases, it starts by changing public opinion.
Having said that, the US is a beast very different from Spain, affording its states far more autonomy than the norm.
Personally I feel this whole comparison between the 2 situations highlights the cultural differences: consensus seeking anglo-saxons, conflict/imposition-driven Hispanics/Latins. (non-hispanic latin here)