Because it is a part of the "Plan". This "Plan" is to His Great "Glory".
But first you need to step back a few steps further and ask why He permitted the Angels to fall ? ...
Why did He permit Satan who was His greatest creation to sin ... ?
How did He come up with the idea for the dumb tree in the garden ... ?
Why did the plan of Salvation before He even permitted Adam's sin ...? Before He even created Adam!!!
If He knew enough to plan Salvation why plant that dumb tree then ... ?
Why did He permit Job to be troubled so ... ? Kill all his kids and destroy him only to raise him up ... ?
Why not admit Angels to the Salvation plan ...? Why are they barred from any plan for their Salvation ... ?
When you get the answer to all of these ... write a book and I will buy your first copy. up