For the host team, it's a cheap date; and it gets to have its way with the other team.
The home team might not sell many single game tickets for these match-ups, but season ticket holders pay for them as much as they pay for conference games. Historically, ND hasn't drawn incrementally when it has brought teams from better conferences to South Bend (Missouri Valley, West Coast, and the like); so I don't really blame ND for scheduling the least expensive opponents. However, the program is more popular locally than it has been in a long time. I wonder if the marginal revenue from ticket sales for better opponents would exceed the marginal cost of scheduling them.
Let's see if there is any extra interest for Ball State, a MAC team. If ND can't get 5,000 people for that game, I don't know why it should bother scheduling more teams from the second tier of conferences.