Why does D2 have a digital optical but no coax interface?

We chose optical over coax inputs and outputs for D2 (and D1) primarily because TVs, Apple TV, and computers all have optical outputs. Regarding optical vs. coax quality, there are various thoughts and theories on which is better but our experience is that as long as optical is implemented correctly and the connectors are high-quality, it is essentially the same digital SPDIF signal.