I think its in one of their stickies about how they select champions.

They release the new rotation on patch days and not on an exact day each week. So whenever we get the patch later this week, we will probably get the new rotation too.

They do rotate the champs, but some of the easier to play, low-IP cost champs appear very very frequently *cough* Ashe *cough*. This is done on purpose, though, so that during any given week new players will have somebody relatively easy to start with.

This does make it difficult sometimes when you are looking to try a particular champion before buying them, but oh well - it does what it's intended to do.