The first reason is Matt Cassel has nothing. Matt Cassell isn't Tom Brady. He can't lead a team to the 2006 championship with his best receiver being Jabar Gaffeney. Cassel is a good QB, don't get me wrong, but he doesn't have anything around him in KC. He had a crazy offensive line consisting of Matt Light (possible HoF), Logan Mankins (best LG), Dan Koppen (Top 5 center), Stephen Neal (3 super bowl rings), and Nick Kaczur (descent player). This doesn't include two good blockers in Heath Evans and Patrick Pass. He also had Wes Welker and Randy Moss. Those names speak for themselves.
The second reason is the loss of Tony Gonzalez. Tony Gonzalez has been the heart and soul of this Kansas City team for the last 11 years. His absense is going to hurt people in the locker room and on the field. He was a great blocker also. He helped two different running backs become the best running backs in the league at that time (Priest Holmes and Larry Johnson). Finally he had hands like a receiver. He could have been a Wide Receiver if he really wanted to. He could be the best Tight End to ever play the game.
The third reason is that they don't have a good secondary. Their secondary consists of Mike Brown, Brandon Flowers, Jarrad Page, and Brandon Carr. The problem is that Carr, Page, and Flowers combined experience (6) is less than Mike Brown's (8). Mike Brown is declining and the other 3 are two young and I wouldn't call Brown the best "tutor" in the world. I would call this secondary not the greatest.
The fourth reason is the ages of the front 7. The 4 man linebacking core (with the exception of Derrick Johnson) is too old. The two outside linebackers are Donnie Edwards and Derrick Johnson. Johnson is probably the best LOLB in the league. Edwards is BIG TIME declining. He isn't the only aging linebacker on this squad. They also have Zach Thomas and Mike Vrabel. All three of these players combine for 39 years of experience and 204 years. The other problem is that the defensive line is led by two 22 year olds in Tyson Jackson and Glen Dorsey.
The fifth reason is Matt Cassel's pressure. Matt Cassell is pressured to lead this team far. First off the Chiefs haven't been very good in a long time. Second Cassel is supposed to live up to the potential of Tom Brady.
I honestly don't think that the Chiefs have what it takes to be a contending team in the worst division in the league.