The problem with the plan was that the government would have had to compensate the power plants for shutting down earlier than planned + pay to import (supposedly coal generated) power from neighboring countries.

The whole thing made no financial or ecological sense.

If you can read french, the official info (with both sides' opinion) is here: https://www.admin.ch/dam/gov/fr/Dokumentation/Abstimmungen/E...

And a german version is available here: https://www.admin.ch/dam/gov/de/Dokumentation/Abstimmungen/N...