The pols in Pittsburg and Philly misjudged how many votes they needed to manufacture. That is the main reason.
Trump and the RNC made a huge effort to get Repub poll watchers in Dem stronghold precincts. When the counts came in for Philly precincts before much of the western part of the state, I suspected that they weren’t able to carry out a tried and true vote fraud scheme, which is for the election chief judges (when sometimes both are dems) to vote many times after the polls have closed. Think about how many times Dem strongholds report their election results hours after the rest of the state. They seem to wait to find out how many fraudulent votes they need. With actual R poll watchers and/or poll workers, that extra voting is not possible.