Simple reason: Most people are inactive, lazy, unfit and object to running.
Get up for a 9am 5k run on Saturday morning? No thanks, most people are hungover, watching tv or drinking coffee.
The fact Park Run's are semi competitive also deters the average person.
For many, once realize they are on the clock, will have their time ranked against the average, and the fact it's an entire 5km, that is more than enough reason to not participate.
Most people 'park' their park run and I don't expect this to change
40-50 people per park run (on a good day!) is all I expect and I live in a metro region with well over 1 million people.
When the weather is poor, Park Run's are lucky to have a dozen finishers.
You'll see more people lined up at Starbucks most Saturday mornings than at a Park Run.