The stated reason was that the games development was not proceeding at a pace that gave the publisher confidence in it. It’s not too surprising after several delays and rather rough looking gameplay material shown in the ramp up to early access.
It’s pretty sad as I was especially excited for the moddability and also as it was the only project the now presumably defunct developer worked on.
But I think it’s still better to cut it off early than to release something that is just not fulfilling a certain standard. It looked like it was much earlier in development than it was from the screenshots.
Really hope the other entries in the genre step in and manage to get the Cities Skylines effect going again.