Why are all the best singers on the xtra factor and not the main show?

Hollie
http://www.youtube.com/watch?v=NXo3qVVFapc

Sophia
http://www.youtube.com/watch?v=iuYOtHL0raI

Katie

http://www.youtube.com/watch?v=bJ2HuD6nqRw

I am puzzled as to why Hollie and Sophia were not shown on the main X Factor. Katie Smith was shown briefly, I think a couple of weeks ago.

Best audition is not shown on x factor
True story.

http://www.youtube.com/watch?v=U9qzYiwsq04