The Spanish state doesn't make any provision at all for secession or independence movements; it's all criminal. They even have a special court for dealing with these matters. Any attempt to campaign for independence puts you beyond the law.
In those circumstances, it's hardly surprising that independence campaigners broke a law.
Because it was one-sided and those not agreeing with it didn't accept its legitimacy, hence not showing up to vote
You don't just put a ballot box outside of the provisions of the state and call that a "referendum"