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".