- Rapid, passive disposal in the unlikely event of a failed spacecraft
- Self-cleaning debris environment in general
- Reduced fuel requirements and thruster wear
- Benign ionizing radiation environment
- Fewer NGSO operators affected by the SpaceX constellation
The first two are because there is more atmospheric drag. I believe that orbital debris in the case of a collision was something that SpaceX was struggling to mitigate (every struggles, but no one has put up this many satellites before).
The third is because originally the plan was to launch to a 400km orbit and then have the satellites lift themselves to a 1150km orbit. Now they intend to launch to a 300-350km orbit and lift themselves to 550km. They expect that the smaller amount of lifting will increase satellite lifetime by 50% even after accounting for atmospheric drag.
The fourth is apparently just "there's less radiation lower, and radiation is bad for electronics".
The fifth is just "less of the other theoretical internet constellations are at this height" as far as I can tell.
(All information sourced from the technical information attachment)