My first fan post, and it is mostly opinion based, so don't take anything personally!
1) The obvious first reason is because Peyton Manning has that offense rolling. He can make mistakes, like he has recently, but quite frankly he's one of the greatest QBs of all time, and at the drop of a hat can make you pay, so you have to go into the game expecting him not to make a mistake. It's not like he's a backup QB like we've been used to seeing recently. The Chiefs leaving points on the field like they have, getting special teams presents from mental mistakes, and just plain being out of position and susceptible to the big play so often on defense, are just things that simply can't happen. The Chiefs have to play a perfect game to beat the Broncos (Which I don't think they can do), whereas the Broncos have more room for error than we do.
2) The Chiefs haven't been good in some time, so maybe some Chiefs fans have never thought of a little term called "Peaked." I think this is what has happened to our defense, and why for the last two weeks they have put up sorry performances on the field thus far. Obviously peaking isn't good, because you want to reach that point as you're going into the playoffs, not 6 - 8 weeks into the season. As you can see, now that they have peaked, they're beginning to come back to Earth, if you need to see examples, rewind through today's game. This isn't the same defense that was playing lights out through the first 6 weeks of the season, they've cooled off, and it is very difficult to regain that type of momentum they were playing with early on. I fully expect them to lay a bigger egg on the road in Buffalo next week, and if they can't snap out of it after the bye week, it won't be pretty in Mile High stadium. It's already a difficult venue to play in due to the elevation, I don't like going to play a 40+ ppg offense, spearheaded by the game's best QB, while the defense is struggling to regain and maintain form. If they screw up early and force our offense to keep pace with Manning, the game may very well be out of hand by the time the 4th quarter rolls around, then it won't matter if they start playing it well (I say this for the people that think that this defense is dependable when we need them, I personally just think that most of the reason you can say that is b/c the game is close enough going into the 4th quarter that they get a lot of trial and error chances to make up for their mistakes). Stop playing the match up games thinking we can go head on against Denver right now, because quite frankly unless the defense reverts back into early season form, we simply can not and looking at paper match ups is simpleton thinking. Quoting the stats that came from the first 6 games of the season as evidence that this defense is still elite, I think that right now, is garbage until the defense picks itself back up.
3) The Chiefs offense still has it's kinks to work out. Admittedly, within the last two weeks they have seemed to be more comfortable opening up the playbook a little more. However, Alex Smith and the offense are still leaving points on the field far too often with over thrown passes, inaccurate passes, that get glossed over b/c he makes one good play a few series later. Example, today Alex Smith overshoots Fasano, and we don't get the Touchdown, later on, he hits McCluster (Who does have to extend to get the ball) and we say he has a good deep ball (Later on he misses Avery up the sideline). Stupid mistakes like not being able to hit the receivers when they're open are mistakes we can't afford to make against an opportunistic Broncos defense (They give up a lot of points, but get turnovers fairly often) that will have Von Miller back into complete football form.
4) This may be apart of the offense, but more centering on the offensive line, or better yet our lack of an offensive line. This line is quite frankly putrid, and I think it more has to do with the guy coaching them up than it does the actual guys lining up (I actually think we should consider firing the offensive line coach). They're letting teams like the Browns and Raiders punk them on regularity, and please no one say, "This is why Alex Smith looks bad." Or something that to that effect, because Alex Smith looks bad even when the line gives him time (Overthrown passes, poor blitz pickup, indecisiveness). I honestly preferred the offensive line of last year to this year's group, it's a damn (apologize for the profanity) shame when you suck so much Jamaal Charles can't even average more than 4.2 YPC behind it. I would honestly argue that run blocking is still more important in Kansas City than completely shifting the focus to protecting the passer (And failing at it). The Broncos defense, already boasts one of the league's best run defenses only allowing 3.4 YPC. So let's go ahead and assume right now that the ground game isn't going to give us anything, which neutralizes our best player. What do we do then to get him the ball? Screen pass it every time to him, so Smith doesn't over throw it if he goes deep?