Why are messenger bags so popular?

I have no direct experience with them my self, but here goes-

-They are waterproof
-They can contour to your body
-They can be as large or as small as the load warrants. Backpacks tend not to be as 'form-adaptive', they can compress the depth, but not necessarily the height or width.
-They are designed to be worn by people who have aggressive body posture while riding
-They are designed for easy load/unload of contents, and make maximum use of interior space. Most backpacks (not all, but especially US designs) have that teardrop shape that minimizes the space towards the top.