Short answer “because it was awesome”?
I suspect that it was a combination of Guns N’ Roses being “new” as well as “raw” enough to appeal to both the 80s hair metal crowd as well as the burgeoning “grunge” crowd who were into bands like Alice & Chains and Soundgarden.
Since their breakup, everyone was wondering when the next “Led Zeppelin” would surface.
Whitesnake was the first band to be christened the new Zeppelin. But that ultimately didn’t work out. And then when G&R hit the scene, everyone was saying they were the new Zeppelin. And… that didn’t turn out be true, either.
Well, Frampton Comes! Alive sounds like something totally different.