If you could take a picture that would help greatly. Pics of the front tires would be good too.
Tires will always wear more on the inside shoulder. If your rears are wearing excessively on the inside and have some feathering, it’s usually from an oversteer condition. I see it a lot on karts that are too soft in the rear. It seems like the kart tends to squat in the corners, lose traction, and slide on the inside edge of the tire.