Moria bridge and gate

[CAB]

I have to think that this question has been asked before, but I didn't see any old threads about it so…

When the Ring-company was passing through Moria, why in the world didn't the Moria forces place a strong guard by the bridge and/or gate? In another thread, Alcuin recently used a good descriptive word for places such as these - chokepoints. Weren't the bridge and gate essentially perfect chokepoints in this situation? The single narrow bridge (as opposed to several, wider bridges) over the chasm was actually designed to be a chokepoint. (It was intended as a defense against outside attackers, but would have worked equally well against those who would try to escape Moria in a situation such as this.)

The Orcs obviously didn't intend to let the company escape. Frodo was speared and hit by an arrow, Sam was injured, and I believe Gandalf got an arrow through the hat.

So why didn't the Moria force choose to utilize the tremendous advantage they possessed at the bridge and gate and instead made the seemingly very poor decision to set the place on fire and confront the company at an obviously less crucial barrier?

I am hoping someone has a good "story internal" explanation. The story external answer seems pretty obvious.