It all comes down to money. Each of the three recent additions, theoretically, bring in money (football brand and market). Basketball can bring in money as well. IIRC, The BTN brings in more money from advertisements than in does subscriber fees. Placing a national brand such as UConn in the B10 can really help with advertising dollars.
IMO, the targets for the B10 include Kansas (I think they go to 20 as an end game). With 2 King programs (Kansas and Indiana) along with several "prince" programs like UConn, Michigan State and Ohio State, the ad money would be tremendous. Throw in solid programs like Michigan, Illinois, Wisconsin, Minnesota, and Maryland, there are great games every week. Advertisers love big games.