Chris Chambers/Getty Images
When the Nets first began, and were still called the New York Americans, before their first season started they were set to play their home games at the 69th Regiment Armory in Manhattan. However, due to pressure from the Knicks, the owners of the building backed out three months before the season started, which forced the Americans to find a new home in Teaneck, New Jersey.
Three months before they played their first game in franchise history and the Nets had already been kicked out of the city by the Knicks. And even when they made the playoffs in their first season in the ABA, the Teaneck Armory was booked, and they couldn't play their playoff game there. The replacement they found on Long Island was deemed an unsuitable environment by ABA commissioner George Mikan, and the team had to forfeit the playoff game.
They bounced around after that to another arena on Long Island, then to Nassau Coliseum, then to the Rutgers University campus, then to their longtime home in the Meadowlands, then to Newark. The team is now going to be playing their home games in Brooklyn.
Up to this point, the Nets have never had the opportunity to build a substantial fan base in New York City because they've never been a New York City team.
This also starts to explain another issue, which is that the Nets just don't have the history that the Knicks do. While obviously it's not like the Knicks have the championship banners of the Celtics or Lakers, they still have been a mainstay in New York for their entire existence. The Nets have not.