Because you can?
Shanahan had time. There were plenty of people to look at, and If he felt that sure that Dubas was the right person, then what he could (and I will argue) should have done, is hear other people's plans. Compare it to Dubas and then decide. What would that have hurt ? A week of hearing other people's plans (hearing from people who view the Leafs from an outside view would never hurt either), and then decide okay i've heard everything, i've heard Dubas now i'm making an informed decision.
I personally believe the best way you find out you have the best is to go through the motions to ensure that is of course the case. The #1 problem with this organization has been entitlement. People just handed things because "it's their time" or "it made sense." So it makes me sad that someone who three years ago preached about taking time, and looking at every avenue into making the team better, couldn't take another week to speak with others to ensure that was the case.
Not to mention if you got someone outside the organization it would be easier to poach scouting and other talent (via promotion) as people tend to bring their own people anyway.
I get everyone wants to be Yah Dubas! and eventually I'll be Yah Dubas! too.
but i would have expected the President of this organization to do everything in his power to get the best person, and I dont think people can argue that we got the best person if he didn't even look further than down the hall.