Pokemon Go vs. Ingress
Tom Vogt

Someone added a good thought on Facebook:

But beyond that, the Pokémon theme affords certain understanding of game elements — most people already understand gyms, capturing, leveling, and Poké Stops are easy enough to pick up as to be trivial.

It is true that PGO uses game elements that people familiar with Pokemon already know, which adds to the point about easy accessibility I made.