I actually prefer the comfort and convenience of handheld gaming over the television even at home. It's nice to be able to flick the power switch, taking the handheld out of sleep mode (I never turn my handhelds off), and instantly resume playing a game. I can lie down in bed, curl up on the couch, or play while cooking dinner. It also doesn't tie up the entire living room which is good if you live with other people.
And I just plain prefer the games on the DS/PSP over the current console games, so that's a big part of it too.