The #1 reason for me was the removable battery. I don't generally need to swap batteries and am nearby an outlet when needed, but I tend to keep phones a long time, ie 3+ years. A new battery is really necessary after at least every 1-1.5 yrs, and a replaceable battery just makes it easier.
The #2 reason was the metal frame. My first smartphone was the Droid X which had a metal body. I loved that phone; it was so sturdy and never needed a case. So, I wanted another phone w/ a similar feel. Ideally I would have gone w/ the 6P, but I didn't want to pay it all out of pocket at once.
The #3 reason, I didn't feel like there was a real competitor to it. Other than the 6P, there was just not another phone that I felt was better than the V10. I came from a Samsung phone and was interested in them but the lack of replaceable battery/micro sd slot and the many, many, many horror stories I read from S6 & Note 5 owners convinced me they weren't options. The Moto X was worth owning but I didnt want to pay Moto Credit a 29% interest rate...yeesh! And sorry HTC but other than front facing speakers, which I love, I can't find much else there I care about. That left the LG G4 as the only other competitor, and I could get the V10 for the same price via Tmobile w/ a $100 online discount.