[pbourke]

That wasn’t the point. The point is people’s illegal enjoyment of the park is actually impeding the legal enjoyment of the park by others.

Some sense of duty along the lines of “I’ll follow the rules in the park even if it inconveniences me” seems to be missing.

[corty]

Maybe that and the sense of duty of the city police.

[webmaven]

I can imagine a few reasons why a leash ordinance isn't being enforced, ranging from the relatively benign "no one has actually made a formal complaint to animal control", through the scandalous "one of the scofflaws happens to be a prominent local citizen, city official, or member of law enforcement" or " the park in question is in a poor or minority-dominated neighborhood", up to and including the entirely tragic "local law enforcement is preoccupied with a rampant violent- and property-crime wave". Most of these imagined scenarios are not mutually exclusive.

I'd bet on some combination of the above like "nobody has even made a complaint in 20 years because 'everybody knows' such complaints are ignored, but the Good Old Boys responsible for stonewalling back then have since retired".