Because if you know a guy is gone in a year, you know that your long term future doesn't depend on him. Some guys will still respect him and give full effort, but some would see him as a substitute teacher and that's not a position that generally gains much respect. It held together this year but I'm not sure it works past finishing out a season. Despite what Rizzi said after the Packers game, it looked to me like the guys weren't giving him full effort after they were eliminated from the playoffs and things started to turn bad in the game.
But, the bigger issue is that his decisions will be for the short term and not thinking about the long term future of the team. And the fact that you may waste a year in building your roster since a new coach may want different types of players at different positions. I'd much prefer to use that year building a roster that the new HC wants rather than a year with an interim coaching looking for one year of success so he has a shot to earn an extension.
I also think it would be hard to sign any free agents on more than one year deals. But, that may not be an issue if Loomis continues to trim the cap.