Fewer potential users in the near-polar regions, so the orbits are angled & aligned like wrapping a ball of twine, while polar coverage would require multiple tracks in north-south orbits.

edit: more answers at https://news.ycombinator.com/item?id=23556843

The orbital planes top parts are all up there at high latitudes. The circle you are seeing is actually made up of several different orbital planes that swoop down toward/past the equator and then up/down to 60 degrees N/S.

Seeing them move will make it clearer.

Here’s an awesome visual explanation with a ton of other cool info:

https://youtu.be/m05abdGSOxY

It'd be super not efficient...

Also, nobody wants a figure-8 demolition derby in space...