The rear tyres on my front-wheel drive Hyundai Tucson City are almost worn out even though it has only done 17,800 km and I expect that by the time the car has done 20,000 km they will be worn out. The car is driven by two oldies, has not been misused in any way, and only on normal roads. It seems to me that 20,000 km is excessive wear especially on the rear that does nothing but rolling with a bit of braking for good measure. I check tyre pressures regularly and the car had a 15,000 km service by the Hyundai dealer.
The rear does more than just roll along the road and contribute to the braking; it actually helps support a fairly heavy vehicle. Most of these type vehicles seem to be getting rather poor tyre mileage and our tyre specialists report similar mileage to that you're getting. Keeping a watchful eye on the tyre pressure to make sure it's up to spec is one of the things you can do, it's also worthwhile checking the alignment to make sure that's in spec as well, particularly if there are any signs of uneven wear across the tread.