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.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.official forums.