Why are “independent” or nonaffiliated candidates not on the primary election ballot?
Some candidates choose to not be affiliated with any political party. We call these nonaffiliated candidates, although they are commonly referred to by voters and the media as independent.
Primary elections are held by parties to determine their nominee for the General Election in November. Offices like Governor, U.S. Senator or State Representative are examples of partisan offices that have primary elections. Since nonaffiliated candidates are not seeking a party nomination, they cannot participate in the primary. This arrangement is state law, not a decision made by elections officials.
Some offices are also nonpartisan, which means no candidates run with partisan labels. County commissioners or judicial races are examples in Oregon. These races do appear on Primary ballots. ~ source Oregon SOS website