I believe that 1. trumps supporters are flimsier and 2. that as trump is nobody's second choice he gets hurt not helped by high turnout.
Turnout was epic, lots of people made up their minds late and showed up, and showed up to stop Trump.
That and evangelicals under-poll and over-perform in IA.
IA is informative regarding Dems going forward (historically) and doesn't mean poop for GOPers.
Bernie needs to put together a serious machine and effort in SC to make a run at it. SC is HRCs firewall.