I believe the overt esotericism/spirituality/mysticism that permeated both their music and their lyrics played a large part in Zep's appeal. That spoke to people. They had one foot firmly planted in the Hippie movement (Purple didn't), yet at the same time already took a step into the future by being something like the hardest-rocking New Age band that ever existed. Though Zep weren't really a hard or heavy rock band, much less a heavy metal act at all, they just had a handful of hard & heavy numbers in their oeuvre.
There was always something misty-mysterious and unearthly in Zep's music, lyrics, billowing production, enigmatic album covers and aloof presentation (which I found incredibly pretentious, but I was and remain in an obvious minority!). It gave room for interpretation. Purple weren't quite Foghat yet, but much more straightforward, in-your-face and of this world. Zep's music is more of a half-painted canvas where you don't know whether the end product will be abstract or naturalistic, but you continue painting anyway, you're in it for the journey.
I think I just wrote something very deep and appreciative of them, I really should receive credit for that.
They still had a sloppy guitarist, a heavy-handed, dragging drummer, a singer stealing lyrics either from Tolkien or black Blues greats (and not paying them proper!) plus a hard-to-hear bassist whose very limited improvisational keyboard skills would not have gotten him a job as Jon Lord's keyboard roadie!