Well, But PJ wasn't dangerous in the way Nirvana was. PJ is great, so these are not disses. I love them. But they were, even then, a pretty straightforward rock n roll band, with some crunchy/grungy guitars, some harder songs (Why Go, Animal, Blood) and some edginess. But there is still a gleam there compared to the uglier Nirvana material. Nirvana was basically a noisy, screamy punk band that had some catchy songs & chill songs that just took hold. "Grunge" seemed to have more underground elements (punk, metal, college/indie music) to it, and Nirvana has more of these qualities in their music than PJ. You can play almost any PJ song for your parents and they won't wince. But try this and see how you fare:
or
There's other reasons of course. Nirvana was considered (somewhat incorrectly) to be Kurt's brainchild, while PJ was/is admittedly more a collaborative band. MTV pushed Nirvana a bit more than PJ. Kurt was also pushed as a bit of odd sex symbol more than Eddie. Sure it's goofy and superficial, but this did sway the impact to the masses back then.