Given that one of the fundamentals of the EU is freedom of movement of workers, workers rights fall right into core competences of EU legislation.
It's just one of those things that always goes in one direction. Central regulation is just piled on.
I'm not saying there should be no labour regulations in EU, just that there is a lot of detailed regulation that is more appropriate to do on a national level where it can take local culture and priorities into account. And the fundamental EU principle of non discrimination based on nationality should ensure that for example a Rumanian worker in France has the same "right to disconnect" as a French worker.
I can't see why Swedes can't decide that they don't mind work emails during the weekend while Germans decide they want to have email free vacations.