Why do digital coax cables matter?

Digital cables are fundamentally different than analog cables.....the cable is an electrical transmission line at 44+ kHz. The 75 ohm impedance match at each end is critical for jitter, minimizing reflections, etc. BNC connectors are made for this, but Canare makes one of the only 75 ohm RCA connectors available......I've tried 'em and they sound better. Any good video cable in between works well, but the cable pedigree is much less important than with analog cables.

Rower30: what you have outlined is my understanding as well, based on Rf transmission theory. Impedance match at the connectors and loss tangent (dissipation factor) in the cable are the two key parameters.....everything else is secondary. That is why cheap CAT5 cable is far superior to most analog audio cable for digital transmission.

Also reclocking the data at the DAC is the key to superior reproduction.
Putting the clock in the data stream was poor engineering from the start. Fortunately, HDMI 1.3+ provides for this and is generally superior to any SPDIF interface, (even with million dollar cables), for this reason. Buy an Oppo BDP-95 and a DAC with an HDMI 1.3 input (Meridian HD621), and you won't have to worry about jitter.