I had the same thought once. Sonic the Hedgehog wasn't even half as popular back when I was a kid when it first came out. I have two theories as to why this is:
1.) The game franchise has grown to include a wide variety of genres and caracters stemming from the simple side-scroller which started the craze, drawing in more fans with each new generation.
or, the more likely of scenarios,
2.) 15 years ago there weren't nearly as many crap cartoons about video game characters, as well as 728065012 sites featuring Sonic character smut. These poorly drawn pornographic images feed the horny minds of stimulation-addicted kids who have grown to love decadence in any form, even pictures of anthropomorphic woodland creatures from video games in sexually compromising positions, and thus their devotion to the game grows as a result.
Younger people are automatically drawn to a classic story of good vs. evil with heroes, villains, anti-heroes, superpowers, mythology, action and adventure. But when you throw porno into the equasion, you've got a run-away train. Basically, I blame Rule 34.