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.

So you are saying there would be little reason to spend $500 on the Bel Canto USB link, for example. Performance enhancement likely wouldn't be that noticeable? I thought I might need a similar device because the soundcard in the Mini is lacking?

The Toslink out from the Mac Mini has very poor jitter performance. It would be worth comparing that to USB from the Mac Mini directly to the Bel Canto DAC3, which has a USB input.

An inexpensive USB to SPDIF converter, such as the HagUsb, may or may not be an improvement over Toslink. Bel Canto's USB add-on gets you 24/96 via USB and, I expect, also gets you much better performance across the board. A device like the Empirical Audio Offramp takes it up another notch or two and leaves Toslink in the dust.