The 6.9L (and subsequent 7.3L) Indirect Injection (IDI) Ford diesel back in 1983-1994 practically kicked off the diesel wars between Ford, Dodge, and Chevy.

Chevy was trying to a year prior in 1982 with their 6.2L diesel but... It wasn't great, to say the least. In 1992 it got a bit better when they turbocharged the 6.5L diesel - but 2001 is when they really entered the competition with the Duramax diesel.

The Cummins was (and still is) a great engine, but a lot of people didn't like how hard it could be to start it in extremely cold conditions, plus it sounded like a tractor due to its engine configuration. When Ford dropped the 6.9L, its glow plug system allowed for easier cold starts and was much quieter in comparison while still being fuel efficient. While it didn't have as much torque as its inline-6 turbocharged competition, it was enough to do what people needed while outperforming Chevys 6.2L offering at the time.

On that note, another fun fact about the 6.9L is that, like Chevys 6.2L, it didn't have a turbo initially - and consequently they were slow as fuck, although their throttle cable and mechanical injection design made them very responsive. They supplemented some of the torque differences from their main competitor (Cummins) by running a compression ratio in the neighborhood of 20-21.5:1 throughout its production run. For comparison, the 5.9L Cummins at the time ran 16.3:1.

After awhile, there was a Banks turbo add-on that you could throw onto the 6.9L/7.3L - problem was, due to the higher compression ratio you couldn't run more than about 10psi of boost without risking popping the head gaskets.

In response to Chevy adding a factory turbocharger to its 6.5L diesel in 1992 that bumped its torque output to 380ft-lb - from 1993-1994 you could get a factory turbo charger on the Ford 7.3L, which increased its torque output to 388ft-lb. There were some tweaks made to the engine design to better accommodate it, although as far as I can tell it still maintained its 21.5:1 compression ratio. There were some inefficiencies in the turbo system that are commonly believed by enthusiasts to have been intentionally not addressed so Ford wouldn't step on their own feet in terms of sales for the 7.3L Powerstroke diesel that followed - although this ( obviously) was never confirmed by Ford.

In summary, the modern diesel war you see today between the Big 3 in the US - where they're all advertising insane amounts of torque and towing capacity out of their diesel engines - is a result of 20+ years of innovation and competition originating from that point in time.