You’re unlikely to get an ‘official’ response, I would expect that Sonos is fairly unwilling to throw a partner ‘under the bus’.
The simple fact is that Spotify needs to give Sonos the legal approval for Sonos to carry their service. Sonos can’t just do it because Spotify is there, they must have legal authorization. And it is entirely possible that Spotify, when negotiating with the local authorities, may have been unable to get that sort of extension…albeit unlikely. But international copyright and carriage laws are rather complex to navigate.
Your best bet would be to start with Spotify, and ask them why they haven’t authorized Sonos to carry their stream in South Korea. I’d imagine the government of North Korea wouldn’t want any extension of a Western company to work at all.