OK, that's not such a difficult question to answer.
Here's the point-per possession performance for IU in conference play:
Some might point to the loss of OG Anunoby as the root cause of these defensive woes, but OG was hurt during the Penn State game—he was on the floor when the Hoosiers were giving up points by the bucket against the average offenses of Illinois and Nebraska.
(Indeed, the only bright spot for the Hoosier D came against Rutgers, aided in no small part by Steve Pikiell's sudden interest in following the footsteps of Coach Knight, the former Hoosier. No, not that Knight. This one.)
So what's the problem?
Well, it appears to be a few things. First, according to Synergy Sports, the Hoosiers are an Iso Paradise. Opposing offenses have had the most success attacking Josh Newkirk (who rates as "Poor"), but Juwan Morgan, Devonte Green, De'Ron Davis, and Zach McRoberts have all been exploited as well. It's not much of a surprise to see freshmen here, but Newkirk and Morgan's inclusion is concerning.The freshmen inexperience also shows up in ball screen defense and rotations. Just an example, but look at Davis' poor angle in attempting to hedge Bryant McIntosh. Not only does McIntosh easily get by him, but frankly, if this wasn't being run out of a horns set (in which Lumpkin dives to the rim), Dererk Pardon has an easy slip/roll to the hoop as well. Just terrible defense, all around.
There are other areas where inexperience hurts, such as rotations and help defense. Here, Davis inexplicably helps a little too much on Duncan Robinson cutting to the hoop (where he's not much of a threat), leaving Mark Donnal (shooting 55% on 3s this season) wide open.
Overall though, this is a very undisciplined defense. James Blackmon is probably the best example of this. The talented offensive player is out indefinitely, but while he's recovering he might want to watch some film, particularly where he can be seen gambling for steals, which often leaves opponents wide open.
And then there are, well, baffling personnel decisions. Michigan has the luxury of being able to run it's 4-out, 1-in offense (and truthfully, the offense is frequently initiated with an empty paint area) with two centers on the floor. This is because Michigan has three big men that can shoot from just about anywhere inside 25 feet with accuracy. What's more, DJ Wilson and Moritz Wagner are capable of putting the ball on the deck.
They are tough matchups. I get that. But ignoring that fact doesn't make it go away. For the first 5 minutes against Michigan, that's what Tom Crean did—he ignored that Michigan had two centers on the floor, and just had one frontcourt defender in Thomas Bryant to guard Wilson.
OG isn't coming back, and it's hard for me to see the team gaining defensive discipline over the back half of the conference slate. Indiana's offense is, once again, very good, so this is still a solid team. But it doesn't strike me as a dangerous, second-weekend type team in March.
Then again, those are in short supply in the Big Ten this season.