maybe a better way of saying it is that he's the obvious choice, because he's popular in Pennsylvania, and it's hard to imagine winning without PA.
And for the record - yes, I don't like the voucher thing but I don't live in PA. My main concern there is that it pisses off the teachers unions. the bigger issue with him is the sexual harassment thing with one of his aides, and the willingness of progressives to stay home plus the risk of alienating Arab-American voters and Lebanese voters in Michigan based on the Palestine thing.
On the positive side, he possibly helps a LOT in Florida, which has overperformed in 3 elections with a black candidate at the top of the ballot.
Speaking of representational choices involving Pennsylvania - in the leadup to the 1964 election against McClellan, Lincoln knew that the support of Pennsylvania and Germans in particular was critical to victory, and chose to promote an immigrant Prussian soldier and Pennsylvanian (and incidentally, a communist) named Alexander Schimmelfennig, despite him being scapegoated in the press for heavy losses at Chancelorsville and accused of cowardice at Gettysburg for sort of laying low at a house in town for a couple of days until things quieted down.
The story goes that when Secretary of War Stanton protested that Alexander Schimmelfennig was unfit to be a brigadier general, Lincoln responded: “No matter about that, his name will make up for any difference there may be, and I’ll take the risk of his coming out all right.”
Of course, he won Pennsylvania, and Lincoln evidently thought Alexander Schimmelfennig was crucial to continued support, because he was later honored to receive the surrender of Charleston.