Amazing how fast Carson is rising.
I think there's a few things going on here: first, evangelicals in Iowa are always strong. Huckabee won in 2008 and Santorum in 2012. Evangelicals love Carson and they have been supporting him for a couple of years. I think some moderate Republicans also like Carson because they don't know anything about some of his more extreme social views, but they like that his tone and persona is moderate, quiet, and intelligent, as opposed to so much bombast from a lot of the others. Like Trump and Fiorina, he has never held public office, and apparently that is considered a plus right now by almost ALL Republicans across the board.
Finally, Carson is black. Very few black men have run for President on the Republican side, and I believe conservatives are sick to death of being called racist simply for opposing liberal solutions to racial problems. The vast majority of conservatives are not racist,
so they are excited by a black candidate who shares their POV.