There was obviously going to be a big dropoff after launch, regardless of how good the game was. Besides that, I think the biggest reasons why so many early players left are:

1. Game was completely unstable for weeks. I can't remember how long it took, but until one of the early "minor text fixes", the servers had potential to crash at any moment. I think this lost them a lot of the "hop-on" players that got sucked in almost purely because all of their friends were doing it/inviting them on walks/etc.
   These crashes were especially bad because they didn't (and still don't!) issue refunds or pause the duration for incense/lures/eggs if they are used during server downtime. Lots of early players got a bad first impression because of this. Issuing refunds would have alleviated a lot of the ill-will
2. Besides server stability, the in-game tracker (a CORE feature of an already shallow game) was completely broken relatively early on. This is an issue in itself. However, there was no way to know it was broken unless you looked for outside information. Players got frustrated because they couldn't find shit with the in-game tracker, and they didn't know why.