Why did Kerry pick Edwards relatively early?

[Nym90]

It was about three weeks before the convention, and was thus a bit earlier than the norm, which has been less than a week before the convention, sometimes not even until the first or second day of the convention.

The pros of picking a running mate early when running against an incumbent President in particular is avoiding being "outnumbered" and having someone else to assist with fundraising and campaigning. The downside is giving the opposition more time to attack and research the VP choice--and of course, waiting longer has the additional advantage of getting a better feel for what help you need and where in the campaign.

Since Edwards had been pretty well vetted throughout the Presidential campaign--remember that he didn't meet Rielle Hunter until 2006--Kerry likely figured that the pros outweighed the cons.