Same as in many other cases - if the two companies have a market presence in a country (or supranational entity, in this case) then their actions are subject to review by that country. The US, the EU and China all routinely use this approach.
Otherwise, to make an extreme example, any two large companies could form a massive monopoly just by domiciling in different countries and each claiming that no-one except THEIR home regulator had the right to review the merger.