Most will say cultural differences but I think the main thing is money and taxes, it's one of the most rich part of Spain. I see it kinda like wallons and flemish people in belgium, 50 years ago wallons were rich and flemish poorer, now it's the other way so it's no more wallons that talk about secede but flemish.

One more thing to know about this referendum is that it's more a PR operation than a real referendum as for it to be legally binding it have to be voted by the 2/3 of the catalan parliament and it wasn't. It only had simple majority as independentist only had a little majority on the parliament.