There are simply too many players that have been successful that don't necessarily meet the ideal characteristics of a left tackle.
Honestly, Brady meets most of them quite well, specifically for zone blocking schemes. A couple inches of arm length might be why he isn't even looked at as a first rounder (with a few exceptions out there) but you are beginning to see over and over and over again executives talking about the tape. His tape is good. His competition was not, so there definitely may be some folks a little bit wary about that.