Complex sentence: The tour is not held every year, mostly because the ice is not thick enough every Dutch winter, and also because about 15,000 amateur skaters may take part, putting high requirements on the quality of the ice.
Split into simpler sentences: The tour is not held every year, mostly because not every Dutch winter permits skating on natural ice. Adding to that, the tour currently features about 15,000 amateur skaters taking part, putting high requirements on the quality of the ice.