Why does my DAC sound so much better after upgrading digital SPDIF cable?

I like my Mps5 playback designs sacd/CD player but also use it as a DAC so that I can use my OPPO as a transport to play 24-96 and other high res files I burn to dvd-audio discs.

I was using a nordost silver shadow digital spdif cable between the transport and my dac as I felt it was more transparent and better treble than a higher priced audioquest digital cable a dealer had me audition.

I recently received the Synergistic Research Galileo new SX UEF digital cable.  Immediately I recognized that i was hearing far better bass, soundstage, and instrument separation than I had ever heard with high res files (non sacd),

While I am obviously impressed with this high end digital cable and strongly encourage others to audition it, I am puzzled how the cable transporting digital information to my DAC from my transport makes such a big difference.

The DAC take the digital information and shapes the sound so why should the cable providing it the info be so important. I would think any competently built digital cable would be adequate....I get the cable from the DAC to the preamp and preamp to amp matter but would think the cable to the DAC would be much less important.

I will now experiment to see if using the external transport to send red book CD files to my playback mps5 sounds better than using the transport inside the mps5 itself.

The MPS5 sounds pretty great for ca and awesome with SACD so doubt external transport will be improvement for redhook cds