During that time The Netherlands was fighting a war of independence, the 80 year war. While it basically stared as a revolt against the Spanish because of infringements of privileges like protection against unreasonable taxes etc.. It soon became also a religious based war between Protestants (Netherlands) and Catholics (Spanish). Since most Dutchmen were still Catholics themselves but against the Spaniards and their inquisition the common idea was that people should not be prosecuted because of their religion. This was specifically mentioned in the act of abjuration (incomplete dutch english translation) send to the spanish king and publicly published in 1581. Besides a certain level of religious freedom several other privileges and rights were mentioned or implied. Like that the prince is there to protect his citizens as like father does his children or a shepherd his flock and not rule them like they are slaves. Also that there should be a justice system that is impartial en equal to all citizens and that there should be a freedom to trade.

All this created at situation that you could hardly be prosecuted for expressing yourself if you stayed within certain limits. Also, the merchant class had become the prominent force at the time which means there are a lot of potential supporters for any kind of art and philosophy instead of a singular/limited source of patronage. All the freedoms created in this situation was the main source that Amsterdam specifically and the Netherlands as a whole entered an Golden Age with wealth and creativity. While not a classical history source an easy to read description on how this came about is Russell Shorto's Amsterdam: A History of the World's Most Liberal City