Because it violates the Spanish constitution as ruled by Spanish courts (https://www.theguardian.com/world/2015/dec/02/catalonia-sece...).

The follow up argument is the government has the power to amend the constitution but it currently has no interest in doing so. If Catalans want to secede they must first convince their (national) politicians to amend the consitution to make secession legal and then hold a referendum on secession itself.

By Spanish law secession is a crime. We can argue if it should be a crime. I think it shouldn't.