With Toews struggling in the playoffs, Barry leaned on Leddy and Mayfield more for those tough minutes than Toews (behind Pelech and Pulock of course).
Lou and Barry probably talked and it wouldn't surprise me if Trotz had Pelech, Pulock, Leddy and Mayfield all ahead of Toews in terms of worth.
If that is the case, and you are going to move a defenseman, it only makes sense to trade the guy that the coach values least.
Covid cap crunch, upcoming expansion, team system, coaching preference, play up until that point, contract, needs for a physical presence on the ice, RHD over LHD, redundancy to Leddy, etc. Toews pretty much lost out by every measure.