Because he is an old white guy with white hair. His role was to balance Obama's inexperience and draw the votes of those on the who are concerned with that.
McCain picked Palin to appease the right wing base with her hyper-conservative views, which is working, I've heard several from the very far right say the best thing that could happen is that McCain get elected and then die immediately.
Then there's the rest of us that see all this for what it is, an attempt to manipulate the system in order to win, to win at any cost. Discouraging to say the least, but after speaking with some history buffs I'm relieved to know things have been done this way from the very start, the quest for the golden ring has always taken the low, crooked road, I've just that I've taken enough interest to see it for what it is.