Thin women are easier to dress, it's just that simple. When something is almost flat, it's easier to make a garment fit, curves are more difficult, that's why you so often see the designers on TV (Project Runway, for example) throw up their hands in horror when they are asked to fit a dress on a plus sized woman. Instead of being mostly flat, she goes in and out.
Another reason is that you need more fabric to make plus size clothes. Fabrics come in fixed widths and the results is that the length of a garment doesn't matter much if you're looking at cost, what does matter is the width. As size 5X can easily take twice the fabric of a small, for example.
The third reason is that most designers rely on basic patterns which have been created for the standard sizes. Those patterns just don't exist forf the larger sizes, so to cater to plus sized petites they would have to make a pattern from scratch, and the last reason is that many believe clothes look better when the proportion of the top to the bottom is similar to that on a 5'10 plus woman.
Having said all that, I think you'll find plus size models are becoming a lot more popular, however I can't recall having seen any who are plus size petite, and that's something I'm looking forward to.