EU countries other than UK and Denmark (who joined much earlier) are both required and allowed to adopt EUR as currency only as soon as they fulfill certain economic/fiscal criteria (don't have the exact figures in my head, easy to search though), whether intentionally or accidentally.
Actually no. The UK is special in that they do not have to join the eurozone unless they want to all other EU members have to join the eurozone once they fullfil the necessary criteria.
Hence.