It comes down to two things: Jeremiah Wright and no early voting.
McCain is reasonably giving up on states where Obama is ahead by 6 or 7 or even more points and where early voting has started. They realize that Obama is banking votes at a high rate - people are enthusiastic to vote and are willing to wait 2 or 3 hours in line. But Pennsylvania is a different story because it has no early voting and is, if you listen to some people, more susceptible to racial and cultural wedge issues. Also, Pennsylvania has a large youth and African American population that might - might - not come out to vote in large numbers. I really doubt that African Americans will find excuses to stay home on election day, but I'm guessing that that is what McCain is counting on, along with a suppressed youth turnout. If McCain can get the polls down to 5 points or less, he has a chance to pull an upset.
Most importantly, there will be - absolutely will be - a stunning barrage of Jeremiah Wright robocalls, mailers, and television ads during the last few days of the election, specifically in Pennsylvania. The media will go nuts but the McCain camp won't care. Frankly, I see this as a poor strategy because if there's any way to guarantee high turnout of African-Americans, playing the Wright card would be it. They will wait 10 hours in line to vote. So the McCain campaign is hoping that they will simply rile up their base, get the voters in the "T" to vote, and flip a few suburban voters who prefer Obama on economic issues but are concerned about the fact that he is African-American.
As a resident of Pennsylvania, I'm excited to vote but am sort of pissed that we do not have early voting. The ground game here is just as good as elsewhere, and if we had early voting we could banking hundreds of thousands of votes already. As a former resident of Ohio, I'm always worried about election-day shenanigans, especially in urban areas and predominantly African-American neighborhoods.
Obama has apparently decided to stop campaigning in Pennsylvania and is focusing his efforts on trying to pick off Virginia and North Carolina and flip Florida. That's fair, but I'd at least hope that Biden and the Clintons set up a tent in Scranton and Wilkes-Barre and take a few day trips to Philadelphia. If Obama wins Pennsylvania, I don't see how he loses.