Living in Barcelona I think this is pretty clear: Tourists. Though what's pizza in NYC is paella and tapas here of course.
Their reasoning is simple. Most tourists don't ever come back so why bother investing in a quality product when you can just milk them for what they're worth. Even reviews don't help with this very much.
As a local it's easy to avoid these places. Avoiding the ones with pictures of the food and sticky promoters outside trying to get you in the door does most of it. And realizing that most of the best places don't have the shiniest venues precisely because they don't need to. The locals know where to find them anyway.
My theory is NYC had better average pizza in the past but people move out of the city over time. They probably move to lower cost areas. Many pizza makers in NJ are originally from NYC. The pizza is better in NJ on average.