Here the word "for" is being used similar to the word "because", or the phrase "because of". It's quite common to do this in written and conversational English. "You couldn’t move for crowds of people." is the same as "You couldn’t move because there were crowds of people." "I can't hear for all the noise." is the same as "I can't hear because of all the noise." Hope that helps!

The area was very crowded. There were many people there.