Maybe a couple reasons??

Waiting for seasons to end for some potential candidates (got to remember, our season ended, but the good teams were still playing).
Also maybe they wanted to have potential candidate list vetted so the new hire could follow shortly after the firing (this assumes we have a competent AD and he has a list)
Lastly, when has ND ever done anything in a timely manner. I would hope they took the time to notify/consult/get feedback from the players in that 17 days.
And maybe that decision wasn't "known" - he didn't get fired after last year, I wasn't sure he was getting fired this year