How did Rash"i's commentary on the Torah become the primary (phrasing intentionally vague) commentary among the hundreds of others that have been written?
Ramban refers to Rashi as "Lo Mishpat HaBechora" -- he has the rights of the firstborn. I think he's chronologically the first major commentary we have ... okay what about R' Saadiah Gaon ... I think R' Saadiah is more strictly translating words? Or is it that they didn't have R' Saadiah Gaon's commentary in Europe?
Simply due to its clarity - short and to the point + covering almost everything