Because they are rightfully frustrated with their current station in life and the direction this country is going, yet they aren't actually educated enough (politically speaking, at the very least) to see that what he is promising is not the solution (and in some ways is the antithesis) to their problems.
I pretty much "skimmed" the debate while at work, and he does play the populism card well, especially for people who don't actually follow politics ie: when he was ragging on how Clinton hasn't accomplished what she's promising in 30 years, and it is time for a "change" and not the "same old same old". Paraphrasing there, but that's very appealing language in an election cycle which has showed a pretty decent portion of the populace is extremely disillusioned with the typical "politician" types. May not be enough to win him the election, but