The reason is that disc brakes do not deliver a measurable performance advantage.

Expecting riders at any level to complete a course faster because they are riding disc brakes is like asking how many seconds faster I'll ride if I change out my seatpost collar. It just doesn't make sense.

Disc brakes do solve several problems, but those benefits are qualitative. Braking is smoother. Rims don't heat up on long descents (more important in MTB). Disc rotors are more easily replaceable than rims. None of these make the bike go any faster.

The argument that one can ride faster into corners if one can count on quick stopping has never been measured. After riding both disc and mini-v brakes for the last year and a half, I've found that the power of disc brakes kept me from finding better lines through corners. Once I focused on flow, stopping was less of an issue. Powerful disc brakes hurt my development as a beginner, in my opinion.

The pros already know this and have skills to match.

A better question is, "Why were no pros riding mini-v brakes?" They are much more powerful than cantis and have been shown to work well in mud too. The answer to that is probably the same one I'm giving here: There's no performance benefit, only a difference in ride quality.

Another thing to consider....the pros have the best mechanics setting up their brakes and are using the best, most specific or custom components out there. They also have the ability to pit every 1/2 lap. Ask them to ride the same bike for the entire race and then you might see a difference in the equipment chosen. It's apples to oranges when discussing their equipment to ours.

Ultimately the Euros would be faster no matter what they ride, and they are much more weight conscious and stick to tradition.  American riders in general on the other hand jump at any new technology that is marketed a being better.

Look how much earlier US riders converted to disc brakes on the MTB, full suspension on the MTB, 29er on the MTB vs their euro counter parts.