Why does the Cetma design clamp to the handlebar instead?
My guess(es) would be:
1) It places the mounting point above the load, which might give advantages WRT stability and the load applied to the mount.
2) Two, laterally spaced, mounts will be stronger and stiffer than a single bolt on the centreline.
3) It ensures a consistent placing of the platform relative to the handlebars - important if you're selling to people with a wide range of stem lengths, but less so if you're building a one-off.
ETA: Looking again, I'd say definately point (2) above. The lower supports are pivoted at both upper and lower ends, so won't resist twisting of the rack about a verticle axis. The rack needs to be braced against this, and the obvious way to do that is to use widely spaced mounts on teh H'bars.