Because it is a round pin, the only way to get more individual contacts is to make the pin longer. If you try to distinguish in another direction you could not rotate the plugs anymore.
Edit: whoops had an old page open
I don't think you can just take the current design and "make it round." You can't just put pins on a cylinder. They wouldn't align as you rotated the connector. The other way to do it would be co-axially like a headphone jack...but those are deep.
