Well… we can only speculate but it seems the it had to do something with Minecraft creators blacklisting some of the servers. Maybe they finally decided to get rid of Offline servers but it doesn't look like that's the case since tons of them are still running.
Was TeamExtremeMC Pay2Win? Not really, but Minecraft EULA is in most cases too restrictive to make it viable to run Minecraft servers.
So if we'd have to guess, the owner probably gave up and didn't want to bother with Minecraft creators anymore. Lucky for you there are still other offline Minecraft servers out there for now!