Because that is the way Microsoft wanted it. The ability to see multiple days in the Outlook To-D0 Bar Calendar has been deprecated as clutter.
One word: MISTAKE!
As one poster put it:
“Why waste all of that space. It makes the calendar to-do all but worthless. It also doesn’t bold dates have appointments?…… PLEASE FIX!”
This worked so well in Outlook 2007 and Outlook 2010, and was not clutter to those that used it.
See THIS Microsoft Blog if you want to add your name to chorus of people asking for multiday calendar view to be displayed in the To Do Bar.