Why USB to SPDIF and not optical?

There is no "valid" reason. It's a question of well the solution is implemented on a unit by unit basis.

Apple has hung its hat on optical and most everyone seems very happy with the results they are getting from the Mini using the best Toslink cable they can buy.

FWIW, a good SPDIF cable - and this is definitely one of those cases where there are real differences between cables - is both more expensive and shorter then either Toslink or USB.