For those guys, dependability in poor conditions is what really counts, and for David Burnett and Nick Ut, Nikons and Leicas delivered the goods. In my quick google search there is a photo of Nick Ut with a Nikon F and a Leica around his neck.
I've not used a Leica, but Nikon F's aren't necessarily all that easy to load either, especially if you happen to be running down a dirt road, with the camera in one hand, the back in the other hand, and the film in your third hand.
It's all in the timing, and anticipation of the next shot, sometimes things work out, and other times they don't.