How are committee members chosen in most state

In most states, committee members are chosen by party primaries.

During party primaries, people can give their vote before the official election date. But, in order to do so, these people had to be a registered members of a certain party. Most states decided to chose their committee members in party primaries because voters typically do not really familiar with the candidates presented for the election. This would give more flexibility for each parties in mixing up candidates that represent its value.