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I think, it is reasonably explained here: https://en.wikipedia.org/wiki/Enlargement_of_the_eurozone

They are required to adopt the Euro, as soon as they fulfill the Convergence criteria Those convergence criteria include "Compatibility of legislation", and Sweden simply doesn't make their laws compatible. (Details here: https://ec.europa.eu/info/sites/default/files/economy-financ...)

In summary, they have to join, if they are compliant. For that they have to pass laws, and they chose not to. So, it isn't quite as mandatory as some people paint it.

To quote from the report:

> The Swedish authorities are invited to remedy the abovementioned incompatibilities

The rules are you have to but when you want, are allowed, or the time is right. Like sweden no one is made to commit to a date.

And Denmark is interesting case as they have opt-out, but they are still pegged to Euro...