It's in UEFA, which is what counts, just like eg Israel or Kazakhstan.
While the "no two games in one city on the same night" rule has been in operation for ages. And I could be wrong, but when two teams both want the same night, I have a feeling it goes to a toss of a coin?
P.S. That chart you cite is seriously misleading if you imagine Paris is bigger than London, "agglomeration" or no.