1. UN/ITU regulates orbit slots/shells (really frequency assignments that effectively limits orbit slots), high decay V/LEO, as in ~500km was basically exhausted by starlink. Big reason PRC announced multiple 10-20k mega constellations a few years ago (without reusable for to put-up payload) was to sign up for next closes shell which is 500+. At those distances orbit decay is decades/centuries. So regulatorily, the fast decay orbit slots are legally mostly gone.

2. All mega constellations including starlink has layers from 500-1500km. Every 100km is like 0.3ms latency but trade off is cheaper station (longer life time) keeping and wider coverage per satellite, but cost more to get there.

Related to 1&2 is this is byproduct of UN/ITU regulations... they can open up more <500km slots, increase congestion, confliction and chance of recoverable Kessler... but that would mean SpaceX (read US military) would... have to share strategic orbits with PRC and whoever comes next.

E: extrapolate to future of cheap space launch, if multiple blocs or even countries want their own mega constellations, and no changes to regulations, then they would have to start occupying higher orbit shells (assuming they follow ITU). Also geometrically, the shell lowest/closest to earth has the least volume / capacity.