Primarily because I spend a great deal of time with a ruck on my back and I can't seem to find a vest that I can comfortably adjust. Especially if I'm jumping. Having a canteen slam into my kidney upon landing is not my idea of fun.
I prefer my slightly modified LCE. I've cut off the padding for the harness so that all that's left is nylon, I find that that allows me to adjust my ruck straps much better, and if it rains, my straps dry out faster. On the belt I have an M16 and a SAW pouch on each side along with some other goodies.
Call me old fashioned, but it works for me and is a good all around solution. If I was tailoring my load for an urban scenario of some kind where carrying a ruck isn't an issue, I might opt for a vest, but then again for us simple infantry types, carrying a ruck is an issue.