I did the research a year or two ago and posted the numbers somewhere, but the Big12 North was by FAR superior to the South for the first 5 years of the Big12.'04 and '05 were the bad seasons for the Big 12 North and somehow that carried over to a narrative that it was always terrible.
Colorado was good, KSU was peak vampire, and Nubs was winning championships in 94, 95 and 97 (Big12 started in 96). OU and Texas averaged 6 wins per year the first 4 years of the Big12. TAMU was the force down there averaging 8+ wins.
The next ~5 years Texas and OU got REAL good w Mack Brown & Bobby Stoops. Meanwhile CU and NU started sliding but were still OK. KSU had a couple 11 win seasons, but then slid too.
The last ~5 years CU went into the toilet and KSU tried the Ron Prince experiment. NU was peak Pellini winning ~10 games a year. Brown & Stoops were still rocking it.
So everyone remembers how it ended, and of course ESECPN pushed the UT/OU narrative a ton as well.