6'4" is the optimum I've heard for DE's but you're right there doesn't seem to be any definitive answer as to why. I would guess it's because they have the ability to put more weight on their frame and that is an immense help in stopping the run. JMO
I found this from USA Today. It's from 2005 but I think it explains it somewhat;
•Requirements: Strength to hold up in the trench wars, speed and agility to aggressively attack and pursue. In other words, a big man with quick feet. The real heavyweights at defensive tackle are counted on to block out the lanes of daylight and in many schemes, tie up blockers that free linebackers to make plays. Ends have duties that vary depending on scheme, too, but no matter the alignment of the X's and O's, pass-rush skills always carry much clout.
Georgia's David Pollack racked up plenty of defensive awards during his time as a Bulldog, but some question his size as a defensive lineman.
University of Georgia