The reason: “You don’t have a genuine down-the-list conservative” among the GOP candidates.
This is an old story and was discussed earlier this week when he said it.
But the quotes above are misleading. He didn't say he wouldn't vote BECAUSE there was no "down-the-list" conservative. His reasoning was much more complicated, and came down to looking at the polls and thinking that a guy he could not possibly support might actually win the party nomination.
The two he said he would have trouble with: McCain and Huckabee.
The man said there was "no down the list conservative". Period, what ever inference you have is your own. The meaning of the statement is that none of the politicians left are conservative.