The first reason that comes to mind is appeal. As unfortunate as it may be, posting pictures of average size women on covers of their magazines, doesn't increase the draw factor, doesn't advertise the products inside and doesn't increase the likelihood of men picking up said magazine.
Often the women on the front cover wear the clothing advertised onside the magazine and a woman who is thin makes the best impression wearing those clothes. There are a few magazines out there who advertise for the average-sized or plus-sized women; but even their covers, on occasion, feature the thinner woman on the cover. Not fair, but it is what it is.
As far ass why don't the designers design clothes for other sizes; the laziness factor comes to mind. A size 1x is not simply BIGGER than a sz 10. There's a proportion difference that needs to be taken into consideration, especially when making slacks or shorts.