But I wonder why Peer pressure was not more effective prior to the leash law!

It is directly related to the amount of times someone steps in poop.  In all honesty, at least in our area, the dynamic was at work prior to any laws being passed. The problem is, that in some cases the process can be somewhat drawn out depending on both the aggressiveness of the person conveying the message and the attitude of the one receiving it.  We worked on some folks for a couple of years.

My observation prior the the leash law was there was very few "inadvertent" unleashed dogs.  They were almost all deliberate.  It was also quite deliberate when dog owners did not pick up after their dogs.

I will take your word for the first statement and agree 100% with the last.