Electronic Entertainment Expo dropped the ball: In 2007, at its very peak of 60,000 attendees, the E3 video game expo organizers decided to drastically change the format. Vegas-style booths and aggressive marketing were replaced with staid hotel rooms and executive-only invitations. It was recognized as a failure, so the E3s since then have tried to get back to the essence of the original expos.
It was too late, though - many game companies dropped the show and flocked to other, more connected expos. One of them was Comic-Con. It's no coincidence that Comic-Con's overcrowding issues started occurring the year E3 dropped the ball.
Comic book movies: Arguably starting with 2008's $300 million grossing Iron Man, companies started shifting their attention from comic book movies selling more comic books to comic book movies selling everything else. Milking IPs goes back to Star Wars, of course, but a modern $60 video game has the potential to be much more lucrative than a $4 comic book. Focusing on video games is just a matter of Hollywood doing the math.
The blurring line between digital comics and interactive games: New iPad-exclusive comics like Operation Ajax require some reaction from the user: Tapping the screen, sliding to move, and so on. Even Marvel and DC Comics digital comics, based on their paper edition, require some interaction. Digitization is pushing the static medium closer to interactive games. And if making digital comics requires programming and designing an app, why not just outright create a game? From the amount of the show floor being swallowed up by games, it's obvious that comic creators are looking to blur the line between comics and games.