http://www.bankspower.com/techarticles/show/10-understanding-stall-speed said:
Unlike gas engines, diesels develop massive amounts of torque (pulling power) at low RPM – enough that it's possible (and, in fact, desirable) to get a heavily-loaded diesel truck rolling by simply easing off the clutch without touching the accelerator. Banks' dynamometer tests show that Ford's 7.3 liter Power Stroke hits peak torque at 1,600 RPM and begins to drop off at around 1,850 RPM, while the Dodge's 5.9L Cummins engine peaks at 1,400 RPM and drops off at 1,800 RPM. As RPM continues to rise, torque decreases even further.
How would you translate this to a torque converter? With a low stall speed. But both the Ford and Dodge torque converters stall between 2,000 and 2,500 RPM – so with a heavy load, the torque converter won’t start turning the rear wheels until well beyond the engine's torque peak. In this case, the stall speed is too high - it is literally impossible to get the engine's full power to the rear wheels! In order to access all of the engine's potential power, the stall speed must be lowered.