Because your pulling the car in a direction its not meant to be pulled and if the strap slips it loosens and then the car is loose. I learned this and was lecured a couple times at Englishtown and Island when I used to run my Comet .... it sounded logical ... and I never had a strap or car come loose. I used to attach it to the rear axle and the front lowers and always had the car stay within alignment. I just replaced my lower rears because when the car was shipped to me they bend the S$%# out of them. It can be done easily with padded nylon straps and not bend anything. If you attach to the frame and hit a bump your oing to rip some thing out on the car or break a strap.
Attach it up near the frame where the bolt goes thru on both the lowers and it will be fine.
I cant say this is the gospil .. it just comes from my experience with towing cars.