No reason why they couldn't have kept both.

The new way is much worse than it used to be when you know the flight number. Put in flight number (before midnight Chicago time) and it gives the right information. That's fine. Midnight in Chicago comes and you put flight number in and get tomorrow's information instead; have to go to "check status of another flight" link, put flight number in again, change default date, click "check" again.

This is all a great deal more work than it would have been if they had simply left the date pull-down on the main page where it used to be.