A rear brake drum lasts longer than a front disc due to physics. In today’s typical front-engine/front-drive vehicle, 69 percent of the weight is over the driving/primary braking wheels. This means the rear wheels are just “along for the ride”, so to speak. Because most of the braking a turning force is in the front end, the front brakes take most of the beating, usually lasting about 30,000 miles. Rear drum brakes tend to last 50,000 miles or more. The addition of brake proportioning valves that try to even out the wear, does very little to alleviate the situation because the front end still has all of the weight.
For more info, see 2 Signs a Brake Drum Needs Resurfacing