I don't quite agree with what Homer6 posted. It's not so much that the adventure game audience has shrunk (or that adventure gamers in general moved over to playing action-oriented games instead) as that more people were able to afford computers that were capable of displaying action-oriented games -- so that people who'd never had any interest in playing adventure games (or turn-based RPG's and strategy games for that matter) were now able to play action-oriented games that were more to their tastes. Meanwhile the market for adventure games remained static. People who'd never heard of adventure games discovered them, while long time adventure gamers grew bored or found they no longer had the free time to play any long games. And some people continued playing adventures as well as sampling other genres.
But the audience for action-adventures grew while the audience for adventures remained constant, so the
percentage of adventure gamers compared to the entirety of computer gamers has decreased, though not the numbers -- and of course the big companies went after the money. Of course there are problems with that strategy. Remember Gordon Aplin's article from 2002 --
***In the Shadow of the Monster*** -- which discusses how big game companies were completely neglecting nearly everyone while focusing on a specific type of gamer. Now the tide seems to have turned with the number of casual gamers far outnumbering the audience for big action games. And it's kind of amusing to read posts from action gamers complaining about how casual games are taking over while there are fewer great games of the genres they prefer being made.
Getting back to why there seem to be more 3rd person games released recently, I think
dtp Entertainment played a big part in the predominance of 3rd person games because of their preference for and promotion of 3rd person games. As far as free games go, it's apparently easier to create 3rd person games in AGS (
Adventure Game Studio) as opposed to 1st person games. It also takes a different kind of creativity to make good puzzles and integrate them into a game as opposed to coming up with a good story and interesting characters. Maybe the people who have that type of creativity aren't usually the same ones who want to create games.
Many people find horror games to be scarier in 1st person, which makes 1st person a natural choice for them (even if the games themselves are more story-oriented than puzzle-oriented). That may be why most first person adventure games we see are horror-oriented.