Why are people allowed to play first time champions in Ranked?

I just finished the last game in my Gold III promos (on my smurf) where our adc picked Tristana for the first time. Everyone else had Mastery 7 and mained their champions that game. However, this guy rushed Runaan's Hurricane and corrupting potion, and lost lane horribly. He did manage to not die much however he was so irrelevant that his damage to champions was surpassed by Thresh. In addition, because of the first tower gold and kill gold they received from our bot lane, they managed to offset the advantage I built top lane, and snowball their mid and jungler. I'm not trying to say that I played perfectly, I could've done better with avoiding Kha'Zix ganks but I'm kind of confused on why people can first-time champions in ranked.