Because I don't see any advantage in delaying the announcement of his vice presidential candidate. If John McCain already has a person in mind for the job, the sooner he introduces the candidate, the better.
Why should he do it sooner?
1) To show that the GOP are 3 steps ahead in uniting the party and getting ready for the general election, while Democrats are still divided.
2) To reassure GOP base early and remove the uncertainty. Of course, he needs to select an acceptable VP for the conservative base for the early announcement to be effective. My pick for VP is Mitt Romney. Once the selection has been announced, conservatives who are still distrustful of McCain on some issues will welcome this acceptable VP pick, and you'll see the money donations start coming more and often to the mccain camp, and the Republican base will be more energized.
3) Because two GOP candidates campaigning simultaneously all over the country is better than one. While Hillary and Barack Obama are still focused on tearing each other apart, McCain and his VP pick can get a head start by campaigning early together and/or separately, to cover even more ground.