[skissane]

In the case of Switzerland, the EU and Switzerland have signed a series of bilateral treaties which effectively make significant chunks of EU law applicable in Switzerland.

Whether that applies to the specific regulations in question here, I don't know – but even if it doesn't, it may take them some time for their lawyers to research the issue and tell them that.

Similarly, for Serbia, a plausible explanation is they don't actually know what laws and regulations it may have on this topic–they probably don't have any Serbian lawyers in-house, and they may have to contract with a local Serbian law firm to answer that question for them, which will take time to organise. Whereas, for larger economies (US, EU, UK, etc), they probably do have in-house lawyers.