"Or, are the people with more complex views (i.e., having both liberal and conservative viewpoints) just less likely to speak out?"

In my experience it's more that people with complex viewpoints get some combination of shouted down by zealots on either side or sick of arguing with ideologues who aren't capable of having rational discussion with an open mind.

Basically the "us against them" message has been sold the members of both parties and prevents intelligent discourse.