A large historical literature supports the generalization in my subject line. Elite troops need collective discipline, to be sure, but they also need to be able to make decisions and take strategic initiatives when appropriate, and masters don't want to encourage their slaves to think for themselves. Slaves are property, not people (and JRRT registers that way of thinking here by referring to an orc as "that" instead of "who").

On top of that orcs just aren't that bright. Hard to know how much of this is "racial" for Tolkien & how much should be put down to centuries of systematic brutalization.

PLUS the orcs have lost their leader in the Chamber of Mazarbul -- the one capable of at least physical initiative that galvanizes his forces.

PLUS (a) the orcs have been disorganized by the collapse of the roof over the corridor, which surely killed many of them and confused the pursuit, and (b) those remaining might be too excited by the fact that they are right there on the Fellowship's heels to think systematically about cutting them off at the pass.

(I'm not entirely convinced by my own argument, but there it is anyway.)