Different tires and widths obviously. The rear end of the bike takes 60+% of rider weight and most of the damaging impacts. I've had or fixed for others hundreds of flats, and all things being equal, you'll get at least 3 times the number of flats on the rear than the front. Other reason is that during riding, the front tire will kick-up sharp road objects, just in time for them to penetrate the rear tire.

Other factor: traction. Unless you're running some kind of weird front-drive bike, you need more tread on the rear than the front.

Same principle with rims and number of spokes. If you have the same gear on the front and rear, then you are overbuilt on the front, or underbuilt on the rear.