I think contestants are chosen on personality rather than sewing skills.
I think they are on all television shows, to get a varied group - who wants to watch a bunch of drones sitting at sewing machines, all doing the same things.
I agree it would be nice if they all had the same level of skills, I feel sorry for some people who can't put a zip in and don't know where to put their darts. But then it's their choice to go on the show, they know there are likely to be more skilled people on there. And sometimes they learn very fast indeed.