Either because they joined before that was a requirement and have a special exemption (what the UK had before they left and what Denmark has), don’t meet the economic requirements, or are Sweden and meet the requirement but haven’t started the procedure which is a weird loophole nobody cares enough to patch… yet.

Nobody really cares that Sweden isn’t using the Euro even though going by the spirit of the law they should, but the U.K. is a much more important economic player that I don’t see the EU allowing to keep absolute control over its currency.