As best as I can determine, the processing plants assumed the grease would fuck things up, so they wouldn’t take them. But after Dominos pizza kept getting shit for producing so much trash, they paid someone to test it, and it turned out it didn’t cause many problems, and the industry has since come around to the belief that it’s ok, and the majority of plants currently accept them.

https://www.afandpa.org/news/2020/afpa-and-industry-partners-aim-set-record-straight-pizza-boxes-are-recyclable-grease-and

That being said, I think it’s still Philly’s official stance that soiled cardboard should not be recycled.