In an attempt to help calm the woes of ranked queue-rs everywhere, Riot is trying out a new change that will make it so summoners can not choose any of the weekly, free to play champions when they queue for a Solo ranked game.

Free2Play Champions are being removed from Ranked Solo/Duo Queue soon unless you own the champions. We may revisit this policy in the future, but we are going to test this out.

- Lyte, Game Designer, via the official forums.

Lyte went on to explain the choice in a little more detail:
In brief, we are exploring various hypotheses we have about sources of noise in Ranked Ladder. Removing F2P champions ultimately raises the barrier of entry into Ranked Ladder slightly (such that you must now own 16 champions to play Ranked); secondarily, players tend to be more experienced with Champions they own, so that might help reduce the fluctuations in skill we are observing.

- Lyte, Game Designer, via the official forums.