In the seventeenth century, the Spanish monarchy crumbled. Why?

Various things. First of all, the crown was in steep financial decline. Unfortunately, they were the wealthiest they would ever be in 1492 BEFORE Columbus sailed. Their empire was too great and they were unable to administer and financially support it. Probably the most important factor was the incompetent leadership that came after the end of the reign of Ferdinand and Isabella. For some reason, the two greatest monarchs in history had a slew of incompetent heirs. That was probably the greatest cause of their strife.