Some of it was the Size. NFL teams were enamored with the idea of a bigger QB.
Of course, you're right. He mentally wasn't ready. He's actually turned it into a schtick so that he can make some money off his failure.
As a Wazzu player, I have full respect. He won the Apple Cup and won the Pac-10. Also gave Michigan a very good game in the Rose Bowl.
I can say that the consensus was truly split on Leaf vs Manning. Of course, it looks obvious now.
Watching that season play out, I was convinced that Manning would win the Heisman. Peyton had been groomed to win the Heisman. And you didn't see too many hybrid players (like Woodson) winning that award.