IIRC, the game states that the temples in their respective areas are the ones that choose the Sages for each temple. So people like Saria and Darunia had no idea that they were Sages at the time because they were not awakened yet. It was only when Link defeated the evil inside the temple was when the Sages were finally awakened. Anyways, here is my reasoning behind why each person was chosen as a Sage:
Saria: Out of all the children in Kokiri Forest, she was the one that was the most knowledgable and connected with the Forest. She knew many things other people in the forest didn't know and even knew information about outside of Kokiri Forest. It is only fitting for her to be Sage.
Darunia: He was a strong warrior that was willing to sacrifice his life for his own people. He is also part of a line that includes a Legendary Goron, so he has deeper roots than any other Goron in Death Mountain.
Ruto: I agree with you on this one. I am not sure why she was chosen to be a Sage other than the fact that she is part of a royal lineage. But I guess the choices were between her and King Zora. King Zora was too oblivious and incompetent to be a Sage though, so I guess Ruto was the next best choice.
Impa: She was the protector of the Royal Family and opened up Kakariko Village for the less fortunate. She was also very strong and had a good knowledge of Hyrule's past. It was so obvious that Sheik knew that she was the Sage.
Nabooru: While she was second in command for a group of thieves, she didn't approve of anything that Ganondorf was doing. She was willing to thwart Ganondorf's plans in order to save her own people from his crimes. So in a way, she was wise enough to look past her position and cared for her people. So her cause was to help the less than fortunate ones where Ganondorf was committing crimes to get his own way.
So yeah, out of all those Sages, Ruto was the one that made the least sense.
