I see this question asked on bulletin boards. Its grounding is something like, "These two candidates are miles apart! How could anyone not be able choose between them?"
Well, I really don't think they are so many miles apart, but, in any case, that distance is totally irrelevant to the issue: Place an anarcho-capitalist and a communist running against each other. Now place a hypothetical voter squarely in the European-style social democrat camp. He's about halfway between them, and may have a hard time choosing. These "How can you be undecided?" folks have confused difficulty reaching a preference for one candidate with an inability to distinguish them, as if the customer in a restaurant who is debating between having pasta and steak can't tell them apart!