Because that addition included OU, NEB. & even Texas A&M. ISU was a regional filler. It was a brilliant move by Pollard to try and be apart of that group but unfortunately didn't work. KU was a part of the group also but they were a filler also. I'm also sure the hope was to get Texas to come afterwards because Texas would think there were no other solutions.
The B1G has to add brands with recruiting grounds. Compare the number of recruits in the B1G footprint to the SEC they are dwarfed! They are to the ACC also but have the better brand value than the ACC. its not about getting to 16 or just adding numbers its about brands that add to tv and secondarily adding recruiting ground footprints for football.
If the SEC gets this done the B1G is left with two options USC, UCLA, Stanford, CAL, Or & UW to entice ND. ND would have all of its old rivals USC,USC,UM, MSU, Purdue as in conference games. The second option is to attack the SEC's grip on the SE with Va, NC, Ga. Tech, FSU and eventually ND(what other choice would they have). This puts the B1G in those heavy football recruits states but they are planted there as the secondary brand to the SEC schools except in NC & Ga.