Lots of good points in here. I agree with MetroForever that this is a problem for almost every team in the area. I heard Fox and CBS broadcasters say stuff like, "We were told it was going to be a raucous crowd here at MetLife, but it's pretty tame." From TV it was easy to spot lots of empty seats for both Jets and Giants games all season. Someone else said the Devils fanbase blows too. That's a pretty accurate statement.
The whole traffic vs train debate is a real issue. Yea people take the train to Yankee Stadium but a lot of those people live within reach of the train. For some reason Red Bull has not been able to capture any fans around the Path or with easy access to the Path. Now with the NYC team a lot of those potential fans have been lost. The easiest or most obvious solution to me, is converting residents of Harrison and Newark into fans. Then move out to other NJ towns where fans could take the train to RBA.
That has not seemed to work at all and the team still seems to be most popular amongst folk in the NJ suburbs. The gameday experience and traffic is horribly foreign to these people and I am sure many had terrible days trying to get to the stadium and just decided forget it.
There's not one silver bullet to fix the problem. Casual fans need to be converted into serious fans that will make sure they get there early and will deal with traffic on the way home to see the team they love. More local fans need to be created. Could they sell $10 upper deck tickets to walk-ups with a Harrison ID? Cap it if you're afraid of losing revenue and want to create demand. As for the suburban families dealing with traffic and parking, that's a tougher one. Are there more parking lots that could be bought? Perhaps it'd be easier to direct fans to a lot a few miles away and bus them to and from RBA. What if they made a "happy hour" where in the 60 minutes before kickoff beers weren't a million dollars each? That might convince people to show up early and disperse the pre-game traffic.
I don't know what ideas are best and which are terrible but those took me about 3 minutes. It's not going to be one massive change that starts filling the stadium. Its small changes that one-by-one improve the game day experience. That will slowly build attendance until the place is noticeably more full.
Then we can complain about how RBA is nowhere close to being capable of handling a sell-out crowd. Out of control bathroom and concession lines that are borderline inhumane. I think we've got quite a few years before were dealing with that more than once or twice a season.