Because being outside and chasing around in all that fresh air was just too damned exhausting for the normally couch-surfing crowd.
PLUS having to keep you're phone on full bright probably caused many to have to recharge more frequently finally taking a further shine off.
It's probably a bit more than that.
Niantic kind of screwed up aspects of the game's design and balance, and they haven't communicated well over issues like server stability and regressing features out of the game.
The game balance issues mean it's relatively easy to get into the high teens, but by mid-20's it's starting to be a significant effort , and presumably by mid-30's is near impossible to level.
Basically people who go hooked by the early gameplay can spend money for a while to keep levelling up st the rate of effort they can afford, but they'll eventually fall off a (motivational) cliff, just a bit later .
eg let's say you can spend enough time at it to gain 20,000 XP per day. That earning capacity is mostly constant, and doesn't increase much as you level up. The cost to level goes up exponentially - eg 22-23 is 100k, but it's over 1 M at35-36. So 2-5 days playing (depending on use of lucky eggs to double XP earned) goes up to 20-50 days. Few people are going to burn cash on eggs, lures and incense for those returns, and their rate of appearance in poke stops is too low to do it on a continual basis by lots of walking. You could get a bike , so you can travel further, but you'll hit the same wall , just a bit later.
They really can't compete with people who cheat, particularly those who cheated early and continue to level with bots (approx 50-100k per day).
For all their experience with Ingress, they have perpetuated the same kinds of design flaws into Pokemon Go, but what works for the hard core nerd audience, does' retain the general population long term.
