I think the large "umdecided" numbers are more a product of our present public education systems: they spend years indoctrinating rather than educating, and yet those few who can read are faced with the dissonance between what they've "learned" and what their minds tell them...
Another factor: Plenty of people who dislike polls give false answers just to mess with them. Then there are those (more than you might think) who say what they think the pollster wants to hear.
Our public education system teaches students how to feel, not how to think. Math is their friend. Science is their friend. 2+2=5? No problem. You are appreciated for your effort and get a sticker.
But there are many home-schoolers, Cruz supporters, who have not yet swallowed hard and made a choice.
Public education can’t be blamed for everything.