#1 - Nike/Phil Knight money
#2 - Draw a 800 mile radius around each school. Which one has more schools to compete against in terms of recruiting, etc.
#3 - Competition - Historically, how competitive has the SEC been on a national level compared to the PAC12? Like Saban said recently in regards to the 12 team playoff, "If 9-3 Ole Miss played in any other conference outside the SEC/BIG10, they'd likely be undefeated and in the playoffs."
#4 - Allocation - I might be reaching here, but different schools allocate more funds to different areas. For example, I would be willing to bet that UK allocates much more money from donors/revenue to the basketball program than Oregon does. And you could even say this relative to other conference opponents.....for example, I'd bet that Oregon might have been one of the greatest spenders on football while they were in the PAC12. Whereas Kentucky likely ranks in the bottom 1/3rd of spenders on football in the SEC.