Well I am a life long ford owner. Dad a Ford Retiree. Currently own a 99 F550 with Pick up bed and 94 Lightning.
240K miles later I have yet to find anything I don't like with the F550. Well maybe the cost of the 19.5 245 70 commercial tires.
I think part of the reason you see so many is they didn't take any Govt money. Lots of folks decided to give them a shot and liked what they saw.
Also you will find thru 2003 these trucks came with the 7.3 powerstroke. Bullet proof 500,000 mile motor.
Lets not talk about the 6.0 and 6.4's :S