Sports Illustrated's Peter King has chronicled Manning's journey as a free agent and tried to answer that very question, though after reading it you still may wonder what exactly was the tipping point for the four-time NFL Most Valuable Player.
For me, the most telling line in the entire piece comes at the end, when King quotes Broncos vice president John Elway, who obviously has some experience in the whole "being a veteran franchise quarterback" thing.
Asked why Manning chose Denver, Elway tells King:
I really don't know. I never asked him. I never asked him who was in it or where we ranked. I just know if I was in his shoes, I'd know how I'd want to be treated. And that's how I tried to treat Peyton.
Forget the head coach or the personnel on offense. Forget the location or the division the team plays in. To me, that quote from Elway is the most telling aspect in this entire story.
Elway, more than anyone involved with any of the three finalists vying for Manning's services, can appreciate all that Manning goes through as one of the most visible players in the NFL. He knows the pressures that are placed on someone who is a face not only of the franchise he plays for but of the entire league.
We may never know exactly why Manning chose the Broncos over the Titans and 49ers. Hell, he may never know himself. But it's clear that John Elway, more than anything or anyone involved, is the main reason Peyton will be wearing a Broncos uniform this fall.