In addition to what the others have mentioned, it seems that the companies changed their production. This applies to Marvel primarily because for a good chunk of their history, the “Marvel house style” gave a lot of input to the artists. In some cases all the writers did was put words in the mouths of the characters while the artists did all the layouts and plotting (that’s part of the reason why guys like Kirby all the way to Liefeld claim a lot of creative control over the work they did at Marvel). That’s not to say that the writer did nothing but a good chunk of the work was left in the hands of the artists. In the past, Marvel sold their books primarily on the strength of their art and it’s safe to say that back in the day, the artists ruled the roost.
After the Image exodus, Marvel downplayed this (for several reasons one of which being that Image took a huge chunk of their market share and were the number 2 comic publisher) and when Quesada came in, he deliberately shifted focus to the writers and started bringing in guys from Vertigo and guys that had done prose without much comic book work. Hence guys like Bendis becoming the top guy at Marvel. So much so that Axel Alonso almost entirely downplayed the role of the artist (which was a very foolish position to take because comics are a visual medium).
Since the Quesada takeover, I can’t think of a single new superstar artist that made the sort of impact the Image guys did. Even moderns day superstars like Capullo and Bennet came up in the 90s (Capullo drew Spawn for a very long time). Most of the comic artists that can sell books came from that era and or worked at Image. Funny thing is, the Image guys- particularly Jim Lee- can still move comics on the strength of their names alone. When Portacio and Silvestri came back to Marvel, they books they worked on were top sellers. But with the emphasis now so much on the writers, I’m not sure those days will ever come back again.