Because tire choice wise believe it or not we have the most options in 16 with 205/45 that can run wide like my Federal RS Pro’s that have a 8.1 section width. To 215/45 16’s from several brands. To 225/45 16 in several brands as well. In 17 we have two choices tire wise stock at 205/40 and 215/45 problem with 17’s is they ride hard. Our cars were never designed for 17’s unibody wise. It was designed around 15’s and 16’s.
15’s selection is ok and the wheels are light but cannot always clear big brakes . 16’s make a great compromise between the two as you can fit big brakes under several models and the tires choices which also improves the ride of the car.
Plus there are several light weight wheels offered in 16 for our cars. Of course Dekagrams are one the premier ones being flow formed i.e semi forged and strong and only weigh 14.6 pounds. need anymore info why?