The parties have already decided their position on a variety of issues so if you're going to get nominated for the party you'll be against them on that issue.

The parties have already decided their position on a variety of issues so if you're going to get nominated for the party you'll be against them on that issue. And the system is designed to exclude independents. The last nationally visible "I" candidate was roughly H Ross Perot. The system made sure that didn't happen again.