Because all keys are printed, build quality (I have an Ergodox EZ and it's ok but not like the UHK), the palm rests are attached, the tenting works better, I like having a proper graphical config tool. I think those are the main ones but I'm not sitting at my desk looking at it, so I might forget something :)
Ergodox EZ has a graphical config tool.
There's quite a difference between the configurator of the ErgoDox EZ and Agent. Agent is a tad more polished, allows multiple keymaps, editing macros, directly saves configuration to the keyboard, restores configuration based on keyboard memory, and allows customizing mouse speeds.