Why are independent voters not allowed to vote in primary elections?

Independent voters are often not allowed to vote in primary elections because these elections are typically organized by political parties, which set their own rules regarding voter eligibility. Many parties restrict participation to their registered members to ensure that only those affiliated with the party can influence its candidate selection. This policy is intended to maintain party integrity and prevent outsiders from swaying the nomination process. However, some states have open primaries that allow independents to vote, reflecting a more inclusive approach.