I think the reason they don't explain much is because a site like space.com would have covered all that many times over by now. Some answers:

1. The copter isn't able to communicate with satellites or earth directly, so it needs to communicate with the rover itself. This is harder without line of site.

2. The copter is all autonomous. Routes are planned and sent for the copter to autonomously fly, because direct control would be infeasible with the latency on Mars.

3. The copter is charged with solar power, and needs time to charge between flights.

4. The main rover has other missions that are independent of the status of the copter. The copter is well well well past the original mission goals, so if it gets left behind that's a shame, but the main rover's missions are way more important.