2004 OSU was the last time a non-KU team made the Final 4 from the Big 12. Since then, Big 12 teams have made 63 NCAA appearances. 2 times Big 12 teams made the Final 4, both by KU (08, 12). 29 times those appearances were as top 4 seeds and since the tournament expanded to 64 teams, over 85% of Final 4 participants have been Top 4 seeds. So since OSU's appearance in 2004, only 3.2% of the Big 12 teams in the tournament made the Final 4. Even in the 46% of those appearances that have been as favorable seeds, only 6.9% of those Big 12 teams made a Final 4. And again, both of those were KU. You could also point out that the Big 12 has only made 2 of the 44 appearances in the Final 4 since 2005, good for 4.5%. 7 of those 11 years the Big 12 has been rated one of the Top 3 conferences in the country and 3 times it was ranked first.
That's a heck of a lot of bad luck.