The major parties were in agreement that son preference in the succession to the throne should be scrapped, but there were disagreements between different political factions about whether other parts of the Constitution were also due for reform. Though I cannot recall the details at the moment, I believe the general perception was that these political divisions turned the prospect of opening up the Constitution for revision into a can of worms that the major parties did not want to open.
Here is the TRF thread which will contain some news reports from the time.
What would it happen if Letizia and Felipe have daughters only? Are they heiress to the throne or is only for males? :huh:
www.theroyalforums.com
ETA: It also helped that the then Prince Felipe and Princess Letizia took the then unprecedented step of announcing in 2006 that their unborn second child was female, a step they stated they took due to the public debate. This was taken as a public reassurance to the politicians that they did not need to revise the Constitution in the near future as it would not make a practical difference in the short term. (Thus, some Spaniards interpreted the premature "gender reveal" announcement as the Prince and Princess of Asturias tacitly promising the politicians they would stop at two daughters so that the son preference issue would not become urgent.)