Why? Because they were BETTER COMICS. Richly writtten, with characters and stories that respected their origins. We CARED about the characters, we cared about the outcome of the plots.
Go onto every and ANY comic book site and say this until you get laughed off the forums. Everybody knows that, yes, though the 90's had their diamonds in the rough (Peter David can do no wrong as far as I'm concerned, even when he was mandated to join in on the universe-wide stories which, unlike today, weren't as well planned), the 90's was the worst thing for comics long-term.
Variant covers, super-duper holographic please buy me covers, Onslaught, Heroes Reborn, Marvel vs. DC, Image "we don't need good writing to sell stories!", the clone saga... it was all about flash. Hence image and the superstar artists. Plots and writing was not what the 90's are known for, it was the pictures and art.
It's because of this that Marvel went bankrupt, you don't see comics at newsstands these days, and MANY (I've heard up to 2/3rds) comic shops had to close down.
No. The 90's brought very little good to the industry. Cherry pick the good stuff all you want, it's undeniable that there WAS good there. But it's not an even balance by any stretch of the imagination.