here's one reason why...Ever heard of Rayleigh Scattering. It's the effect of different wavelengths of light travelling at different speeds through any medium, the glass of a fibre included.
This doesn't cause any probs if u use a perfect mono-chromatic light source, but realistically any source of light, even the red from a red LED, contained a band of red light accross a narrow frequency range.
Therefore, some red light will travel down the cable faster than other frequencies (can't remember if its lower freq the faster or higher freq, but dont really matter when describing the effect). What happens is at the reception end, the square digital pulse "spreads out" in time.
This makes it hard for the reception circuitry to decide when EXACTLY is a 0 or a 1 transistion, and thus adds JITTER. JITTER as we all know degrades sound quality in digital systems, and this is well documented elsewhere.
Therefore, quality of optical cable is impotant. You want the purest glass to minimise spreading, and the best approximation to a monochromatic source you can get, sadly not the cheap red LEDs commonplace in most players.