They'd seen what Eisenstadt had done at LIFE with the Leica, wanted to do the same and, one suspects, they were tired of hauling all that weight around. Oscar Barnack's asthma really did revolutionize photography in that respect. Plus, for newspaper work, negative quality no longer mattered, at least as far as grain goes. As long as the grain was smaller than the screen used to print the picture, nobody cared. The XL still has its uses, of course.
Negative quality is still very important when shooting weddings and other similar formal events.
A fast focusing camera is still important in those settings, and interchangable backs would also have obvious value. I find it's great for shooting family stuff, too. I do believe all upcoming press photographers should be required to use an XL, or a Speed Graphic, when they start out. These cameras teach discipline, how to look for pictures, how to wait for the right moment, and how to make each shot count. See photos of the Entire XL Line Brought to you by Graflex.