I'm a little surprised that the VP candidates have not already been named. Time is the most precious resource any campaign has. The presidential candidates have endless demands on their limited time. There are always forums to speak at, fundraisers to attend, interviews to give, and battleground states to visit. As long as the VP has been well vetted and won't pull a Dan Quayle, there seems to be very little to gain by waiting. The VP is the candidate's most important surrogate. They can attend the lesser fundraisers and events that the candidate is unable to attend.