Because he could die...

In the West Wing during season 3 , the idea was tossed around that they could replace the VP John Hoynes because he won't be able to bring in his home state of Texas. The discussion is ended by Bartlet deciding that Hoynes will be their guy "because I could die".

In the real world, with all the other considerations such as ideological balance, bringing in another state, forcing your opponent to campaign somewhere they had taken for granted, for selecting a VP, the oldest and indeed the original reason for the position is that the President might die or be killed in office and then the vice president has to assume the office and the quite rightly awesome responsibility that goes with it. We live in a pretty strange world and all too sadly a world that has some pretty strange and violent people in it. For that reason, both John McCain who could just as easily succumb to disease and Barack Obama because some nut job might decide he must die, the vice presidency selection of both parties may be more important to the election process than at any time since a former shoe salesman from Missouri joined the Democratic ticket in 1944.