I think Fassel is a decent coach, but the team just doesn't seem to have the proper chemistry to put together a dominant season and playoff run.
Bad team chemistry will usually get a coach fired, after all these are "his" players. A new coach can come in and make the necessary personnel changes, not to mention the needed change in attitude, without recieving the criticism a long term coach would. As we so often see, even a good coach can many times only get a team so far, and a change is then called for. Tony Dungy, Dennis Green, Mooch, etc.
"Beer is proof that God loves us and wants us to be happy."