Because current X implementation is completely screwed up.
Instead of rendering on server and sending only high-level messages like "here be buttons", X's rendering everything on client — even rasterizing fonts, then sending bitmaps over network. Obviously, volumes are huge so is latency. And there isn't even a slight hope of look-and-feel consistency, too (how client on remote machine is supposed to know your GTK theme?)
Wayland may seem weird, but it could be a step in a right direction. Just throw in network transparency at widget toolkit level.
The typical thing is to run the X server locally and the client remotely. There are lots of reasons to do this such as the server possessing special hardware, license managers, fast disks with the data you care about, more memory, more cores.
For my usual desktop tasks, all the 'data I care about' is on my very own hard drive. Also, processing power of my laptop is good enough to not need any 'special hardware' or 'more memory' than I have. That is why I asked why Mr. John Desktoe Ubuntu-user would want to have network transparency. (I realize that this might be different for some bigger installation, but most (Linux-) people just run Ubuntu locally on their Laptops)
The example user might like to use some word processing software, some native email client with big storage, some graphic editing and maybe even some Twitter app. Some people prefer native, installed applications to doing everything in a browser. What do you know, she might even want to do some programming!
Laptops are so small and powerful these days, why would I not run X locally?