The fact: Russia (the former USSR) has launched much more rockets than any other country. The current statistics shows that the Russian equipment is the most reliable! Therefore, cargo can be carried to the International Space Station by any foreign space ship while people can be delivered only by the Russian space vehicles.
However, we will not go into details. Being great pioneers and groundbreakers, astronauts are still really superstitious. That is a paradox. We should knock on wood and it is not meant as a joke.