The platform direction is determined by the primary process, not the presidential election. For the election the candidates are trying to engage with whatever groups are on the fence to get them to swing towards them - the aim is the middle.

During the primary candidates don't have the luxury of the middle - they need to find a large enough group of support that can push the party in some direction that's going to get widespread party approval while trying to find a candidate that still has a chance at winning the election.

If you don't want to vote for president then at least go vote for the many other things that will be decided that day!