They aren't even on the same depth chart. Sandin is a LD, Liljegren is a RD. I guess the Leafs could jump through hoops to fit Sandin in the lineup over Liljegren when Liljegren has outperformed Sandin in the AHL and Sandin has not consistently proven to be an NHL capable defender at any point in his extended NHL stints.
Not saying Liljegren looked great in most of his 13 game NHL career to this point, but he has certainly looked more refined at the AHL level (vs. Sandin) and he looks like a guy who just needs more than 13 games in the NHL to adjust. Heck he was pretty good in his two games with the Leafs this year, although he was in a very sheltered role so that should not come as a surprise.