Or, perhaps because undecideds are truly clueless, thus McCain is just living the mantra that issues don't matter:
http://rossdouthat.theatlantic.com/archives/2008/09/what_the_media_covers.php/snip
The McCain campaign's decision to lie about, well, everything, really needs to be understood as more than the outcome of John McCain's consuming ambition. It is a rational and obvious response to the rules laid down by the media. Indeed, McCain's spokesperson Brian Rogers says this directly to The Politico's Jonathan Martin. "We ran a different kind of campaign and nobody cared about us. They didn't cover John McCain. So now you've got to be forward-leaning in everything."
And it's true. Earlier this year McCain made poverty tours and offered policy speeches. No one cared, Obama retained his lead. It was only when he began offering vicious attacks and daily controversies that he began setting the pace of the coverage. The McCain campaign learned something important about the media: It's an institution that covers conflict. If you want to direct its coverage, give it more conflict than your opponent. And so they have.
* * *
Undecided voters don't think in terms of issues. Perhaps the greatest myth about undecided voters is that they are undecided because of the "issues." That is, while they might favor Kerry on the economy, they favor Bush on terrorism; or while they are anti-gay marriage, they also support social welfare programs. Occasionally I did encounter undecided voters who were genuinely cross-pressured--a couple who was fiercely pro-life, antiwar, and pro-environment for example--but such cases were exceedingly rare. More often than not, when I asked undecided voters what issues they would pay attention to as they made up their minds I was met with a blank stare, as if I'd just asked them to name their favorite prime number.
The majority of undecided voters I spoke to couldn't name a single issue that was important to them. This was shocking to me. Think about it: The "issue" is the basic unit of political analysis for campaigns, candidates, journalists, and other members of the chattering classes. It's what makes up the subheadings on a candidate's website, it's what sober, serious people wish election outcomes hinged on, it's what every candidate pledges to run his campaign on, and it's what we always complain we don't see enough coverage of.
But the very concept of the issue seemed to be almost completely alien to most of the undecided voters I spoke to.
/snip
My take is that there is a chicken and egg situation here. Perhaps the undecideds are clueless because after eight years of a Bush administration that has made no effort to explain how the federal government affects or screws up their lives, people have no understanding of the issues that affect them. They have been conditioned to being uninformed, and the solution is not to ignore the issues, but to empower people by informing them of how issues affects them, and why they should take ownership in their government.