Maybe she doesn't have enough accomplishments and experience yet to win. On the other hand, Malala Yousafzai won the Peace Prize when she was 17, just two years after surviving the assassination attempt against her.
From the article:
“I think we have reached a tipping point where enough scientists are telling it like it is and not being so afraid of being alarmist,” Thunberg told New Scientist on Wednesday, adding it’s been frustrating that many people are paying more attention to her age than the issue at hand.She's right about this, too.
“They talk about our age, our looks and so on,” she continued. “The emissions are still rising and that is all that matters. Nothing has happened, that is crucial to remember.”