are you serious. How about because of the same reason why 99% of all businesses in this country do what they do. MONEY. Plain and simple. Besides who would they put in if they didn't take OSU.
Lets look at it
Orange Bowl- Has to take an ACC team (Va Tech)
Sugar Bowl- Has to take an SEC team (Alabama)
Rose Bowl- Takes Big 10/ Pac 10 team (PSU and USC)
NCG- Takes #1 vs. #2
Fiesta Bowl- Takes a Big 12 Team (Texas)
Allright we also know that a conference can not have more than 2 teams play in a BCS game in a year. So that eliminates, Texas Tech, and Okie St.
So basically what we have left is TCU, OSU and Boise State as possible selections. And be real, if your on the selection commitee who are you going to take. TCU's home field holds 44,358 people, Boise holds 32,000 people, Ohio State holds 102,329 or 25,971 more fans than the other 2 combined. Not to mention that TCU only filled 68% of its seats for home games, so they don't even care about their team when they play on campus. Also there has been 42 BCS games played all time. The game between Boise St and Oklahoma 2 years ago, while it turned out to be one of the best games ever had the 3rd lowest rating of any BCS game ever played. So clearly people nationally don't want to watch Boise St. play football. Conversly, OSU has played in the 4th, 5th and 6th highest rated BCS games ever. There is a very clear and simple reason why OSU got the nod.