The 5-foot-8 midfielder scored a goal and notched an assist for three total points on the season. (The San Diego Union Tribune)
I thought the preposition in usually collocates with season. Why is it on the season here instead of in the season?
The two most likely prepositions in that context and meaning would be for the season and on the season; the reference is to the full duration of the (playing) season, the season in its entirety.
They had three flat tires on the drive across the country.
This phrase is only used with sports, meaning the total points scored during the sports season (until now).
You can't say "on the season" to refer to other kinds of seasons:
(wrong): We've had three snowstorms on the season.
(right): We've had three snowstorms during/in the season.
(also right): We've had three snowstorms this season.