the dark knight rises - Why didn't the prisoners who helped Bruce try to escape from the pit themselves?

Indeed the other prisoners (or some of them, the younger ones maybe) still try to escape from the pit, they just don't make it. There is a scene where one tries to escape while Bruce is still lying paralyzed.

Now the doctor could tell the others the "secret" of escaping (which I'm not sure he ever did). But in the end, just leaving away the rope is not enough, it is more of a mental stimulation. You still need the will and hope to escape and you still need the physical abilities (while Bruce was getting older and had his problems physically, he wasn't one of those hopeless failed existences imprisoned for years down there).

Even if the doctor had ever told the "secret" to the others, they probably were just too hopeless and desparate to try the climb without the rope and risk the rest they have of their lives. And even if they did, they either didn't bring the needed physical strength or, even more probably, the needed mental strength and the will and determination to escape and survive, having their will and hope broken after such a long time in the pit.

In the end I'm not sure the doctor was that aware of this "secret" of escape at all. It wasn't just about leaving away the rope, you still need a will as strong and determined as that of Bruce. And only when seeing such a character in Bruce the doctor realizes that there is finally someone after the kid who could make it out of the pit and he gives him the final stimulation he needs.