This is a subject that could go on forever..and I'm not sure there is a correct answer. Probably a hundred correct reasons is more like it. I do,however,think there a a couple of primary reasons.
Obesity is at an all time high,both adults and children. Physical activity,such as hiking in a park,may not have the same appeal that it would have 40,50,60 lbs ago.
The entertainment choices today are mind boggling. DVD,MP3,Hi-def,Xbox,on and on...and kids just absorb this stuff. I saw a kid about 9yo,with his mother at a store the other day. This kid gets out of the minivan and walks into the store with his mother,never taking his eyes off the gameboy he was using. It was kinda sad to watch actually. Playing outdoors becomes a low priority,because the allure of all the high tech "wow" toys keep kids indoors.Hiking and camping just aren't "glitzy" enough for kids fed on a steady diet of fast moving special effects and action.
Maybe it's a generational demographics issue. The boomers who grew up on "go outside and play",were more likely to have a higher percentage who kept their interest in outdoor activities. The boomers are now maturing,and reaching a point where many cannot,or no longer want to engage in those outdoor activities any longer.The generations following,have frown,and are still growing up on a diet of "hi-tech",multi-media and graphics-indoors. Are they so disconnected,that those outdoor activities don't occur to them,or have no appeal?
I guess what it comes down to is competing for attention,and video games and Disney are winning.