Why are the worst suburbs across state lines?

[Linus Van Pelt]

With the exception of Gary, all the places mentioned are at a distance that could plausibly be within the city limits except for the fact that the city center abuts a river which forms a state boundary. So it may be not so much that the state line actually causes suburban areas to be poor, but rather that the state line makes essentially urban areas to count in some technical sense as "suburban".

[Linus Van Pelt]

With the exception of Gary

Actually, Newark is far more of an exception than Gary. (Now... if the OP had referred to Jersey City...)

Newark is about equidistant from Lower Manhattan as East New York or the south Bronx, so one can imagine a hypothetical NYC that went as far into NJ as it actually does into NY including it. That's all I meant to be getting at.

When I lived in NJ the train I would take into New York stopped at Secaucus Junction after Newark (sometimes delayed there due to the bottlenecked rail tunnel under the Hudson whose expansion Chris Christie cancelled) so I am unfortunately well acquainted with the industrial swampland along the Hackensack.