It cannot be definitely answered in a manner that would even approximately explain all countries reluctance to adopt euro. But I have to point out a few things:
- You are mistaken that there is a "ERM II loophole" - it's true that every joining EU country must adopt euro, which starts with joining ERM II,
- but there is no time limit on petitioning to join ERM II
- because after entering ERM II joining country must fulfill a number of entry criteria, of which Poland (our example) fulfills none. So there is no point in forcing anyone to join euro if they won't make it even past initial stage.
EU requires that every country that joins adopts euro as a currency, however except for joining and staying in ERM II for two years there are no other requirements, including time limit to do that and weren't at the moment of accession of Poland. There was no need for one, especially since EU itself preferred that when Poland joined EU. And introducing any requirements after that would be a breach of the Treaty, obviously.
ANd when crises hit it was easier and safer to just shelf the issue for better times.
I can only explain why Poland doesn't want euro - and it's at the moment what matters most. It's in addition to the above, and this will be mostly from memory (however based on very lively public discussion on the issue back in the good ol' times - that is before 2009):
- It would require changes to the Constitution, which is hard to do in PL
- It would greatly reduce the role of the NBP
- It would limit the government ability to borrow money to finance deficit.
- It would reduce country's export competitiveness. Traditionally high exchange rate meant relatively low prices of polish goods in euro countries (mainly Germany)
- ERM II exposes countries to speculative attacks on currency, which are very expensive to defend against.
There was a reason for the open end. in 2004 eurozone was in much better shape, even if there was a number of people and organizations that warned that what we see right now will be the final outcome of the euro currency project. The countries participating (or at least most of them) made most of the requirements and it was them who did not want to rock the boat by introducing countries with fiscal and financial issues. And some countries weren't keen on euro in the first place. Poland is a big exporter so most of the companies even preferred to exchange currency to euro - mostly because we sold a lot outside EU and this wouldn't change after adopting it. So it would be mostly political decision which public would not appreciate at that time to the point of upsetting the political stage. And no one wanted that - both EU and major political parties in Poland ATT were wary of the opposition (times proved them right, it seems).