Leafs opposition have figured it out and used systems (neutral zone trap, and defensive zone box with all players collapsing down low and boxing out HDSC etc etc) with the use of player personnel that are big, physical, defensive oriented and designed to increase their goaltenders ability to see pucks and not face dangerous rebounds in close.
CBJ and Montreal both successfully systematically made their goaltenders look good and jobs easier, because they know highly skilled players like Marner love to stay on the perimeter where they can avoid physical contract, and avoid the high danger areas including down low in the blue paint, because they're not willing to sacrifice physically to take hits to make plays and go into heavy traffic to do so.
With a muffin for a shot Marner is never going to beat a goalie from distance, particularly if its unscreened, and with no traffic down low. So they leave him out on the perimeter, taking away his main passing option Matthews and essentially nullify him from being a dangerous scoring treat both himself or this linemates but clogging up passing lanes. Break up the rush and Leafs preferred attack and easy zone entries, and force dump and chase tactics which their skilled players are ill prepared for to regain possession is already advantage defense.
Keefe's only answer to his successful playoff style defense and shutdown systems being deployed against the Leafs is to play Marner more, like that is going to get him to magically change his style of play with more icetime. If you can't beat Columbus goalie wait until you're facing Andrei Vasilevskiy or other elite goalies as the underdogs in a series and see if the results change in Leafs favour.
Then the excuse is always the opposition goalie keeps beating us and keeps shutting us out and that is simply bad luck, but eventually that will change for Leafs. Not until the Leafs core players grow a pair and a backbone and willing to play in heavy traffic and take the abuse to change and breakthrough that system ,nothing will change only the names of the opposition team and the next goalie.