Because 1080p on a 5 - 6 inch device creates visible pixels to the naked eye, and can be improved upon with a higher quality screen.
I have a 1440p 5.5" smartphone and the difference next to 720p is staggering and the difference next to 1080p is still noticeable to the untrained eye. The tests I use to demonstrate to people include well formed text display, comic-book display, and Unreal4 demo. People pick out the 1440p screen as best without much issue in every test.