Because, there is a LARGE contingent of Leftist Republicans who vote...
And, because in “regular” polls where only a small sample of actual Republicans vote, Jeb ain’t doing so well.
He needs help from the Left.
Romney was able to win so many primaries with the 30% support the GOPe has, since the 70% conservatives in the party are too dumb to ever hold a conservative convention and unite their vote choosing 1 candidate.
Bush isn’t doing near as well as Romney because he has so many RINO’s in the race too.