I think it's the explosion of Multiplayer online gaming. With Game consoles having networks that "find the game for you", no one wants to play against scripted AI computers. Games that have an MMO element certinaly is not going to have bots (though I understand your comment about training). I think before when LAN games were popular along with Internet gaming, you found more bots because friends wanted to team up and maybe you didn't have enough players to make a 12 player match.
That said, At my work, we still use bots for simulations and testing (but the entire match is BOTS, no Humans are playing).