The orcs at the west end weren't on guard for things coming out of Moria,
because they were on guard for things from Lorien. So the Fellowship
essentially "snuck up" on them. Besides, they are orcs and not very
intelligent!
I think the forces of evil were so involved with the chase that they didn't
think about running a different way and cutting off the escape. It
takes someone/leader to direct the chasers to go in different directions
instead of just running after their quarry. So, since the orc chieftain
had gotten killed in the chamber, and other orc leaders might have gotten stuck
behind Gandalf's shutting spell (or they were Balrog food), there wasn't anyone
to direct the
pursuit.
