Because amps are significantly louder now, and feedback is an issue with unpotted PU's, as it is with hollow bodies. That's why these days almost all PU's are potted, and the vast majority of guitars are solid bodies.
Question for ya... The '59 LP PAF's were not potted, right? Or were they? So, does Billy Gibbons '59 have potted PAF's? If it doesn't why are the "Pearly Gates" potted?