I think the BTN FAQ they answered that question:
This is reprinted from Snowman's post in the BTN Strikes again post.
Each week, Big Ten Insider will answer a frequently asked question. We kick off this recurring feature by answering the two questions we are receiving most.
Q: Why doesn't the Big Ten Network televise every Big Ten men's basketball game?
A: There are several contributing factors. In college athletics, the home team or home conference holds the television rights to its events. The Big Ten Conference only holds the rights to events hosted by a Big Ten university. When a Big Ten team plays a non-conference road game, the hosting university, conference or tournament organizer holds the television rights for those games and thus decisions already have been made regarding how those games are televised.
For basketball, the Big Ten has divided its television rights among the Big Ten Network, CBS and the various ESPN networks. The Big Ten Network will televise more basketball games than any other network and the Big Ten's television partners will combine to nationally televise more than 200 Big Ten men's basketball games this season. No conference will have more games televised nationally than the Big Ten.
Q: Why doesn't the Big Ten Network have overflow channels for basketball? [I have DISH/DIRECTV, and my online channel guide always lists Big Ten Network alternates.]
A: The overflow channels are only available to the network during the 13 football Saturdays of the season, which are finite windows during a time when there are few other sports taking place. The non-conference basketball season overlaps with many other sports, and so NHL, NBA and other regional football and basketball games are also taking place on the weekend and on weekday nights when basketball games air.
During the conference season, the Big Ten schedules games in such a way where there is no overlap. Consequently, there is no need for overflow channels during the conference schedule.
DISH Network changes its on-screen channel guide template infrequently, and so the alternate channels are listed, even though they are not available to us except during the football season. DIRECTV will change their template this week to remove the Big Ten Network alternate channels from the on-screen guide.