Geographically, these countries are located on the European and African continents. This is a difference with the US dollar, which is also used by a few countries outside the Western Hemisphere. The intensive trade and financial links with the euro area are the main factor behind the choice of the euro in the definition of exchange rate policies.
For some countries, the European Union accession process provides an additional impetus to select an exchange rate arrangement based on the euro. I should highlight that the choice of a euro-based exchange rate arrangement is a unilateral decision, and does not involve any commitment on the part of the ECB.