Because he is slow. He played Mike for Philly and he really is better at it than Will. Edmunds wanted to play the Urlachler role for Eberflus so since he fit the physical profile the Bears overpaid to bring him in.
The Will is a the smaller faster player, Bassa will be a much better Will than Edwards or Sanborn.
Edmunds will be fine if they finally add a real DT tandem in front of him. Your DTs should the best players in your 4-3. The Bears DTs are the weakest link in their defense.
Right now COVER 2 and.other 2 DEEP variants are the most commonly used defensive playes. Even with a different DC your going to need a LB like Edmunds to drop the pipe C2 and man up in a 2 DEEP Man.
If your running any even front 1GAP you need a DT tandem like Tank and Tommie where BOTH DTs can get to the QB and BOTH DTs can consistently shut down the run.
Instead of the current DT tandem where Billings is good against the run but nothing special as a passrusher and Dexter who gets a handful of Sacks with QB pressures while Sweat is doubled off the edge, and is a bowling pin against the run.
This defense needs a significantly better DT tandem.