Why do digital cables sound different?

Rja - AFAIK jitter is the only difference between digital cables. I don't know of anything else that can affect the sound. Jitter can be measured but many factors can change it - like level of electrical noise around, amount of system noise etc.

One cable might work very well, having perfect impedance matching, with fast slew rate outputs while the other might offer perfect shielding - important with slower transitions especially in noisy environment. In addition there are preferences of flexibility, color and overall quality. There are, of course, identical performance cables offered by different companies - same as with analog interconnects.

Why do you want to establish absolutely the best ($$$$)? My system and my ears are perhaps not good enough to appreciate the best cables (analog or digital).

I would still trust my ears or reviews more than measurements. Don't spend money if you cannot hear the difference.

It would be fun to ABX the Kimber D60 (almost 1k at 2m) with the Blue Jeans Belden 1694 $14 dollar or whatever cable. I'll have the Blue Jeans one, anyone got an extra D60 they can lend? He he. And don't misunderstand, I'm a Kimber fan overall, definitely. (FWIW, and I think we aren't supposed to talk about this, shhhhh, but I do 'believe' in ABX. I tried it with spoiled milk and normal milk and got it right 10 out of 10 times.)

10 out of 10 on spoiled milk test ain't bad - I got 6/10 on the milk and 7/10 on rotten eggs.

I don't necessarily need the "best" but it seems that if the level of induced jitter is the primary criteria for judging digital cables and if this can be measured then these cables should much easier to select. Unlike other cables there would be much less subjective opinion to deal with and things could be simpler with less trial and error.
Digital cable reviews could read; 20 cables tested, here's the 3 with lowest jitter. Then the only things to choose would be termination and color. Maybe wishful thinking. 8^)

Rja - Cable itself does not have jitter. It creates jitter by reflecting fast transitions on impedance boundaries or allows slower transitions to be contaminated by less than perfect shielding. It is impossible to test it without standard ambient noise source (and location) and standard output transitions (each transport is different).