I think they siad, in the first movie that they didn't want to use characters just for the sake of it. They only viewed the Big 7 as worthwhile because of their familiarity to the movie watchers. Since they could not think of anything more than a cameo for Chapel I suppose they did not want to pay an actress for a 3 movie deal if they were not going to get their money's worth just to please fans.
What's frustrating as a fan is that I can think of quite a few ways they could have used Chapel, Rand, and a few other characters spread across both movies that would have added dimensions to the plot without making any major story changes. I think they could have added more value for money than the drawn out and convoluted CGI monster scenes or a threesome with a couple of hairy hookers but you can't please all of the people all of the time.