Dubas was only interested in signing or acquiring goaltenders who have experience playing in a tandem, and Mrazek has done so for years in Carolina.
Jack Campbell is likely to start game 1 next season for the Leafs, but Dubas wanted to bring someone in to push Campbell for the crease and someone who was comfortable splitting time regardless of how well you're playing. Mrazek has done this and more for the Hurricanes. He's also shown he's capable of taking over the net and being the guy, which was very appealing for Leafs management.