Part of it is the Devils get buried by the other hockey teams in the area in major media markets (Philly and NYC x2), so the Devils get lost in the mix. You also have a lot of people in NJ who are fans of the Flyers, Rangers, or Islanders (or even the Bruins to a lesser extent) because those teams have been around much longer than the Devils have (NYR and Boston are original 6 teams; the Flyers have been around since the 1960s IIRC).

It sucks, but it's also kind of understandable. That said, we were good in the past in the 90s/early 2000s, but the past 10 years have been painful to put it mildly. Thankfully, all of those years of sucking, being in denial, and then rebuilding are finally coming to a head this year and we are a legitimate playoff team now.