Because they spend about 30-45 minutes of the program doing stories on the teams involved. It's a free commercial.
"IF" NU was 3-0 and hosting, you could guarantee that ESPN would talk up the new sports facility, the rich tradition, the top notch athlete services, Team Jack, etc. It would have been great, if it were to be. It would do nothing but give NU some much needed national attention before they entirely slip away from the spotlight.
Plus, its 3 hours of the entire country seeing that NU has a very loyal fanbase.
Among many other benefits.
Not hosting isn't the end of the world, but to pretend it doesn't matter is also kind of juvenile...