Ever wonder why Bryant Park and Union Square are junkie-free but not always Washington Square Park?

That’s because New York University, which owns most of the buildings surrounding the park, has a love/hate relationship with the community.

In the early 1980s, junkies and the homeless were splayed all over and sandboxes were filled with cigarette butts, glass and worse.

Today, things have changed — but not by much. As The Post recently reported, crackpipes are now busting out all over.

Yet the similarly (and maybe worse back then) junkie-infested Union Square Park and Bryant Park are now infested with millennials and food markets.

The other city parks were able to clean up their acts because the business and real estate communities were so appalled at the state of the city that they banded together, and with the blessing of Mayor Rudy Giuliani and City Council legislation, they formed Business Improvement Districts.

When the city wanted to renovate Washington Square Park in 2005, neighbors filed lawsuits and held up the redesign for several years.

After it was completed in 2009, the Coalition for a Better Washington Square Park wanted to hire additional security officers to keep drug addicts out, but the city nixed that proposal.

Rather than a BID, which would involve ongoing support from the adjacent property owners — read NYU — the neighborhood formed The Washington Square Park Conservancy.

With a 2015 budget of $265,000, it supports the City’s Parks Dept. by providing some horticulture, maintenance and grants to support cultural programs — not security — although it works closely with the local NYPD precinct.

NYU, with an overall school endowment in 2017 of $4.1 billion, made a one-time $500,000 contribution, said Dr. Gil Horowitz, a retired NYU professor and neighborhood activist.

“We think they could give a lot more,” he confided to The Post.

[Post-publication, NYU, in a statement, said it committed $300,000 in June 2005 for renovation of the park arch, plus another $1 million two months later for general park renovation and maintenance. In addition, NYU provided nearly $1 million over the last four years to fund park safety officers and has ponied up $10,000 to $15,000 a year to support programming in the park, it said.]

NYU does contribute “generously” to the free park concerts, but Horowitz added, “We don’t want them getting too much control. We welcome their help as long as it doesn’t come with, it’s their park.”

Horowitz praised the Conservancy’s executive director, George Vellonakis, who designed the park. “He is doing a credible job at keeping the park safe, clean and green,” Horowitz said.

And Horowitz, who is in the park almost daily, says that while drug dealers had been defiant, with police cameras everywhere, they now stick to their corners. “You don’t see them shouting, ‘coke,’ just ‘smoke,’” he explained.