I think one factor in this election is that the parties splits are misaligned with issues that people actually care about.

So for example: Trump and Sanders (and Tulsi and a number of more fringey candidates) are generally opposed to international intervention.

Immigration considerations are another one (in particular, dem immigration platform is at odds with trade union preferences but aligned with large corporate interests).

There are big, relevant, issues that voters care a lot about that don't conform to the party lines.