Because a new IRC client without widespread protocol extensions means everyone currently using IRC will hate people using the new client due to the fact there is no place to hide metadata.
Not every new feature has to be a protocol extension though? Some clients just lack some sugar coating client-sided features. As for protocol extensions, you could always just disable them in-client on servers that don't support said extensions if anything. But my focus is on making more cleaned up IRC clients, a lot of them feel like they're stuck in time when I know there's lots of things that could be done to them to both make them user-friendlier for newcomers and make them a little more convenient to use.