He was one percentage point shy of carrying the plurality in SC. In a general election, GOP voters who did not support him would have a choice of wasting their vote, essentially electing Hillary, or picking the lesser of evils. I realize some would refuse to vote for him, but a lot are not that stubborn.
He has a lot better chance of being elected than Romney or Huck. Which is a big part of why he is doing so well in the primary. His poll numbers against Hillary are impressive, even in some blue states, where he is currently ahead of her.
There's that old electability argument again. How could those polls mean anything when the majority of those polled only know anything about those candidates who have received heavy media coverage for years. Whoever the Republican nominee is, he will receive enough media coverage to change any perceptions or non perceptions that are registered by these early polls. Also, see my earlier post on "electability".