The thing is, Zeppelin and (to a lesser extent) Nirvana weren't really that huge when they were originally active. Stairway in particular didn't make the Billboard top 100 chart in either '71 or '72, losing slots to timeless acts like The Partridge Family and Hot Butter.

Teen Spirit was Nirvana's biggest single, and the most obviously pop-inspired of their songs that I know of. It hit number 6 at its' height and number 32 at the year-end chart of 92. Good, sure, but it also lost out to "I'm Too Sexy" by Right Said Fred, and Patty Smyth and Don Henley's Sometimes Love Just Ain't Enough.

Musically, as I've said, Teen Spirit is a 4 chords song that can be picked up by anyone, and Stairway was so overplayed by guitarists that it's still a running gag to this day. Great songs, absolutely, but there wasn't anything particularly special about them other than that a certain generation remembered them and passed them on to their offspring and basically ensured their legacy.