Twisted Treeline ranked not playable at certain times

Dear Riot, Today i was excited to get on with my 2 friends to play twisted treeline ranked, only to find out that we arent allowed anymore. Even though we an play ranked on summoners rift with the three of us, we arent allowed to play twisted treeline? i think this is a real shame, and dont understand the reasoning behind it. Just wish we could play TT ranked. Thank you for reading and could you give the reasoning behind this decision? Thank you.