No clue. I would guess that it's a big factor for parents purchasing handhelds for their kids since they're probably thinking "This'll keep him/her occupied and shut him/her up during errands and trips", haha. Kids usually don't have a ton of games, though, and they get so excited to play them that I can't picture them "saving" a game for a trip out. They'll play when and wherever they can. And adults lose opportunities for outdoor gaming as they get older, like how people already mentioned that they're the driver now and such. Plus some adults may want to avoid the stigma of playing a "child's toy" in public. (Your mileage may vary with that depending on what handheld you're playing, how old you are/look, and what your gender is. I feel more judged as an adult female gamer since I'm seen as more of an oddity than an adult male gamer.)
While I guess it's an aspect of the portability, I more so love the comfort of not being tethered to anything. Even playing a home console with a wireless controller isn't quite the same. I can sit or lay in just about any position I want and still get a good view and play. I can also have the TV going at the same time or whatever. It's much more relaxed, comfortable gaming, and it's works better for spending time around others than being absorbed in the screen of a home console game (outside of multiplayer gaming, that is).