Why can optical usb cables not operate below 0°C?
What is the physical reason behind this?


Probably, there's no specific physical reason for exactly this range.

It simply costs significant money to test electronics, and 0 °C – 45 °C is a common range for consumer electronics certifications. If you're an industrial customer of Corning, I'm sure they'll be willing to offer devices certified for a larger temperature range, of course, for a price.

But think about it: What USB equipment exactly do you have that is specified to work below 0 °C? I've yet to encounter one consumer device that has that certification. Hence, it's very unlikely a user of the cables would ever be able to complain about the temperature range – simply because the printer/hard drive/SSD/camera that he'll be using with it isn't certified for freezing temperatures, either. 

If there are physical reasons, they'd be:


electrolytic capacitors not being happy about freezing
voltage converters used to power the internal electronics changing reference voltages, leading to malfunction
laser diodes might not work great with reduced carrier mobility
mechanical properties of the rubber/silicone tubing, plastic case making it e.g. more brittle
increased chance of condensation/thaw on the device


The optical properties of the fiber optics almost certainly have nothing to do with it.