Philadelphia is a large port city and is still one of the biggest cities in the US. NJ sits between two major metropolitan areas that are a two hour drive from one another.

As a result, Philly and NJ had a lot more factory industry back then. Towns like Camden and Paterson, that lost to decay and crime now, used to be prosperous factory cities. A lot of NJ’s industry still benefits from its proximity to New York. A lot people that used to live in NYC but now want a yard for their family move to NJ. Large companies tied to NY also do things like put their R&D parks in NJ because it’s close to NYC but has the space they need.

Bell Labs, GE, IBM, Philco, RCA, and other bigtime tech labs were spread across the [upstate] NY to Philadelphia region. These areas were expensive, unionized and had issues with racial integration. SV until the early 1960s was not run by union bosses, was cheap, less regulated, and still had exceptionally racist white enclaves with solely single family housing, so there was less worry about integration problems.