Tell that the the Manchester United supporters who bailed on the club when the Glazers took over, and created a new one club.
http://en.wikipedia.org/wiki/F.C._United_of_Manchester
Also, a couple of supporter groups for Red Bulls teams have created new clubs after Red Bull completely disrespected their heritage by changing the colors and/or name of the club.
So yeah, its not just about location. There's a large number of reasons why a person would support a club. Tradition is a huge part of it...and thats a large part of why there are NYers who kept allegiance to RBNY or chose them as their team even though they had the choice of NYCFC. Does location matter? Sure it can...but heritage, tradition, and culture are very important too.
Talk to some soccer fans living in Brooklyn or Queens. Before NYCFC came around there were some people who were drawn to the NY Cosmos. I grew up in Queens, and if the Cosmos were an MLS club, they would have been my first choice over NYCFC. Why? Because they have the longer tradition of the 3 mens clubs in metro NY right now. I was drawn to that tradition, and for someone who grew up in eastern Queens, Hempstead is easy to get to.
I live upstate now, and itd be a pain to get to a RBNY or Cosmos game. But the reason I dont support RBNY is because the club never grabbed me emotionally. Part of it was location, and part of it was their branding and colors. People underestimate how much branding and colors matter to American sports fans...especially when certain cities like NYC have firmly established team colors across different sports.
Plus some NY soccer fans have been resistant to NYCFC because of the "mini Man City" fears. Sounds silly at first when you consider other Red Bull soccer teams around the world....but the reality is that no Red Bull soccer team is as visible to the world or as popular as Manchester City. So for NY soccer fans, being a RBNY fan might feel more unique and independent to them than being a NYCFC fan under the City Football Group umbrella.
And the reason I didnt latch onto the Cosmos is they are second division, and really never had much outreach to the fans. Its hard to get latched onto a team you never hear much about and dont see a lot. Location is a part of it too, seeing as Im so far away from that area now. But if the Cosmos were in Queens, I could imagine them having a larger following. And an even greater one if they were an MLS team and had the bigger marketing to go with it.
To summarize, location does matter...but so do other things as well. The NFL is the biggest sport in the nation and both NY teams play in New Jersey....but neither have identity or fan issues.