\Why did the civil rights movement become increasingly assertive and militant over the course of the 1960s?

Slow government response

Explanation:

At that time, the massive discrimination toward women and minorities group run rampant in the country. Causing a massive problems such as difficulties to obtain job, significantly lower wages for the same work, different treatment in government court, etc. But the civil right movement that has been started since early 1900s still pretty much ignored by the government.

This caused many activist groups started to emerged in order to shake up the status quo. Some of the groups are militants but some of them are peaceful but assertive.

One of the most famous figure that arise during this era was Martin Luther King Jr. He led several peaceful protests that managed to attract national assertion and forced the government to responded. This led to the creation of  the civil rights act in 1964.