I have a problem with my Hyundai Tucson 2007, the rear left brake rotor is
wearing only half an inch from its center (inside and outside face). I have
checked the left rear brake pads (in and out), I cleaned the caliper and the
caliper mounting bracket, the pad retainers and the brake pads still rub
only on the inner brake rotor. The brake rotor is rusted 1 inch from the
outer edge on the inside and the outside. What could be the problem?

 

Are the rotors original? You probably need to have them cut or replaced. I suggest replacing them and replacing the pads.

 

Rear disc brakes with e-brakes requires a special tool to turn and push the calipers back in so that one can put new pads on.
If that wasn't done then it can cause the caliper not to function properly.
But, usually, uneven wear is caused by uneven pressure applied to the brakes
Which can be caused by the slide/guide pins being stuck.
Remove them, clean them and grease them and re-install