The Big Ten, as do all the conferences in NCAA athletics, has gone through a lot of changes in the last year. The conference added four new members with Washington, UCLA, Oregon, and USC joining from the Pac-12, which, as of right now, no longer exists. The addition of these programs brought the total number of teams in the conference to 18, which means there are changes when it comes to certain conference tournaments.
The Big Ten basketball Tournament is set to begin in a few weeks, with just a few games left in the regular season, and while in the past every team made it in, times are changing and some teams are now fighting for a spot.
With there now being 18 teams in the Big Ten, the conference chose to make a change to the Big Ten basketball Tournament and instead of having every team make it in, now only 15 of the 18 teams will make it into the tournament and have a chance at running the table and getting an automatic bid in to the NCAA Tournament.
With only 15 teams making it in, that means that the top nine teams will receive a first-round bye and the top four seeds will receive a double bye. So the bottom three teams in the conference after the regular season concludes will effectively have their seasons ended early and won't get a shot at the Big Ten Tournament title.
This is a huge change compared to another conference where every team makes it into the conference tournament. Last season, NC State showed that sometimes playing your best football at the right time is all that is needed to make a run. The Wolfpack made a run and ended up winning the ACC Tournament for their automatic bid to the NCAA Tournament, a bid they most certainly wouldn't have had without a conference tournament win.
NCAA State then went on to make it all the way to the Final Four in the NCAA Tournament, something they never could have done had they been allowed to participate in the ACC Tournament.
No one really knows why they chose 15 teams instead of just keeping the tournament at 14 games or 16 games and making it an even number, but 15 is what they chose to go with.