Because he limits what Minnesota can do. He provides too much of a risk for Minnesota to gamble on. If he has constant nagging injuries that sideline him for a game or two throughout the year, we can't place him on the injury reserve, which means we're going to have 4.6 million tied up in cap space that we can't recover. I forget how IRs work in terms of recouping, but I know it can be a PITA.
In season IRs are 10 games a 24 days missed for an injury. Broken bone, muscle tear, major injuries basiclly and a player can be list the next day. Consussions are usually a retroactive thing after a guy has missed a couple of weeks and isn't looking like he'll get back within the next 2 weeks. The problem is they don't save you any cap space. It just means you can got over the cap by that amount of money without it hurting the team's yearly cap.
The best example is the 14-15 season when Kane got hurt bad and went on the IR for months. The Hawks were against the cap, but with the Kane on the IR they could trade for Vermette, make callups, and be over the cap without penalty.
The salary cap is actually figured on a daily basis, and then all the days are added up to make the final cap. The numbers can be fudged by sending players between the AHL/NHL on off days. It's how teams with $1m of listed cap space can go and get a $4m player at the TDL (rough example).
Last year the Wild had daily cap hits ranging from ~$70.6m (bye week) to ~$74.7m (after the TDL).