It's pretty simple why; because they make a shitload of money. 3D films, particularly animated 3D films have been some of the top grossing films around the globe over the past few years, so much so that the 3D gimmick has come back in a big way and has been started being applied back to horror films as well (the My Bloody Valentine remake, the latest Final Destination film, etc).
I for one actually kind of dig it, because 3D technology has come so far over the past decade. I mean man you compare the old Red and Blue-box paper glasses that were used for decades with these nifty little sunglasses you get to go see Avatar, they're just on different planets. One of the things that made Avatar such an enjoyable experience obviously was the added 3D element to go along with the insane CGI work from Cameron's studio, because along with the fantastical story of the film the 3D really immersed you into the story and made you feel like you were in that brightly colored and beautiful world of Avatar yourself. I wasn't sure if I was going to buy into all the hype for Avatar and it's 3D elements when I went to go see it, but when I did it really did add a whole new dimension to the film-going experience. We may be looking into the future of film ladies and gentlemen, another chain in the evolution of filmmaking. At first there was only silent black and white, from there we added the technologies of sound and later color and even later HD and anamorphic Widescreen use, it only seems logical that the next step would be the inclusion of advanced 3D technology to finally once and for all actually attempt to immerse you into the film itself. I for one am absolutely thrilled and excited as hell to see what the future of 3D technology holds in filmmaking.
But, to answer your question more simply and to end my quick little rant on my hopes for 3D, the reason why so many films being released now are in 3D is because they make a SHITLOAD of money with the added gimmick. With the oversaturated market of animated Pixar-esque films for children, they had to add in the gimmick of 3D glasses to seal the deal for parents bringing their children to these movies. Kids like it, it gives a film an advantage in the odds of some people's eyes when going to the theater, and it isn't going to be going away anytime soon man.