Pretty much that you invested that much time and effort into getting to a high level, it's tough to just say "I'm done".
My kids play some, they have lost a lot of interest as well though so usually I have to initiate the "let's go wreck some gyms", but they get into it once we get out there.
We prolly have 20 pokeballs between the 3 of us and walking around downtown or the local park crammed with pokestops doesn't sound like much fun in December so hard to find much desire to play.