I just didn't like the OS from a usability standpoint. The email client on my WP8.1 worked much better than anything I could find on Android. Now I'm stuck using Oulook in WP10. And I understand Outlook for Android is even better than WP. Cortana sending, reading and replying to SMS messages in my car worked flawlessly under WP8.1. It's become very spotty since upgrading to 10. Automatic quiet hours based on calendar appointments in WP8.1 was an outstanding feature that is completely gone in WP10.
I guess the bottom line is that I'm ready for something new and more customizable. Android has come a long way in 3 years and I believe I can adapt more easily now. And yeah, the app gap is catching up to me. I probably won't ever use Snapchat, Pokemon Go, etc, but I'd at least like the opportunity to try a new app when it comes out.
And new hardware! Many responses here have said "wait and hope something new comes out". How much longer will that be? I'm done waiting for that elusive unicorn. Yes, the new HP Elite X3 is an awesome piece of hardware, but not on my carrier.