1) Penta pixels:
I've gotten used to true 1080p on the display of my 1+1.

2) Lack of confidence in OnePlus:
My 1+1 radio has a tendency to suddenly "fall of the cliff" in areas of modest to weak signal strength. Something ain't right there, and lots of others have noticed the same issue. Generally, a reboot is required to reacquire signal again.

3) The Nexus 5X can be obtained cheaper than a 1+3 now.