And it worked: X-Men #1 sold over 8 million copies and inspired like 8 million copycats as everyone jumped in on the multiple covers bandwagon. With so many variants everywhere, the editors had to think of ways to make theirs stand out, and that's how we got infamous gimmicks like holographic covers where you could barely tell what was going on ...
Marvel Comics
... glow-in-the dark covers, black-and-white covers, covers with all sorts of fancy embossing effects, covers with holes cut on them, covers with "magic eye" illusions, covers splattered in fake blood, covers with plastic trinkets attached, and, because they knew their audience so well, comics that came pre-sealed in plastic bags for collectors, which meant they had to buy another copy if they wanted to read the story (but at least those usually came with stickers and other useless junk inside).
At the same time, Marvel raised prices and began churning out more and more new series to keep the company growing and make it look more enticing to investors. They also hyped the hell out of their superstar artists ... which came back to bite them in the ass when Lee, McFarlane, Liefeld, and others left to form their own company, Image Comics. And of course, they took all of Marvel's gimmicks and dialed them up to 11 -- or higher, in the case of the new series that came out with 13 variant covers, including a blank "draw-it-yourself" one.
DC Comics
DC wasn't quite that shameless, but they did start relying way too heavily on "special events" to keep sales high. After killing Superman and crippling Batman went pretty well, they felt like every major DC character had to go through something traumatic: Green Lantern had his city nuked and became a villain, Aquaman had his hand eaten by piranhas, Green Arrow had a helicopter blow up on his face, Wonder Woman was fired from Wonder Womaning, Flash stubbed his toe pretty bad going to the kitchen one night, etc. And, for every savvy comic book reader who realized that these were all just temporary setbacks, you had nine newcomers who were sure that Superman was really dead forever and his "final" issue would fetch millions sooner or later.