The main reason is simply that it so happens that most of the very top universities are based either in the UK or the US. (The fact that ETH Zurich is the only non-UK/US university that is in the top-20 on both the QS and Times Higher Education rankings partly reflects this, although my sense is that these rankings have some pretty serious limitations and should be taken with a major pinch of salt.) I also think there are additional benefits associated with attending university in the UK/US, including in terms of opening up career opportunities in the English-speaking world.
I agree that ETH has some things going for it and including it might well have been a reasonable choice, although my impression is that its teaching language at the undergraduate level is German, which means that it’s not really a relevant option for the vast majority of potential applicants.
In general, the decisions about which universities to include involved a number of debatable judgement calls, so I think there is a decent amount of room for reasonable disagreement on this topic.