Shawn was around for every era in WWE in some form or fashion since the Hulkamania era. In that frame of time he became the archtype of what you want your progression to look like in terms of buildup as was Bret. Shawn however, has the Ladder match, the first Rumble victory coming from #1, the first hell in a cell, the first elimination chamber, helping usher in the Attitude era with Austin, etc. Cena is def top 10 but you cant credit THAT much to him. So looking at it from the point of view of someone in WWE, Shawn has done more.
What else, IN WWF ONLY, puts Andre there over Savage, Bret, Shawn etc.