Because even as an outsider in Washington the fact he's a Senator makes him in insider and republican primary voters' minds. That's why Trump, Carson and Fiorina are running 1, 2, 3. No stink of Washington on them, though it sounds like she was involved with Washington after HP so the right negative campaigning can tie her to that and sink her.
The question is whether this will persist through enough of the primary season to get more voters for those with no political experience than those with, and we'll end up with a brokered convention. At least that will make things interesting, though while I hate to admit it looking at those three I'd take another Bush any day.