Due to form factor and size.. 720p for power/performance and compatibility. That res on a screen of that size good enough for most users and going 1080P would be a bit overkill. Also would drain battery more considerably.
Fyi a 720p screen would be 209.8 PPI. This is almost double of that of a 27" 1440P monitor. Granted that you'll be viewing it closer, this is a good sweet spot for portable performance.