Because like the neo-fascist Lombard League of northern Italy, they are a rich region of a nation that does not want their money going to the poorer regions of that nation. That is it. Sorry, this is not about language suppression or anything else. Catalalunya/Catalonia achieved autonomy on education, health, and local law enforcement a long time ago. The only thing they lack is all that money they are sending to the rest of Spain.
Yes, they will probably get to keep their money in the future. I thought it was conservative Spanish PM, Mariano Rajoy of the PP, who stupidly ordered national police to attempt to break up the independence referendum, who engaged in violence of shooting rubber bullets, clubbing people with billy clubs, and dragging people by their hair, leading to hundreds injured, although nobody dead. This order that will probably lead to full Catalunyan independence was ordered by a Spanish national constitutional court judge who had ruled the referendum illegal. If he really thought he was going to stop the vote, he is an idiot. All he has done is guarantee that "reasonable people" will support the 90% outcome of the vote, with only a 42% turnout, given that opponents apparently boycotted it. But like those who did not want Crimea annexed by Russia, boycotting a vote organized illegally by those who want a certain outcome, only plays into the hands of those who want that certain outcome.
The path to this has been a mess. The referendum was voted for by the current Catalan parliament that has a 72-63 members split between those favoring independence and those opposing it. Like the US, Catalonia/Catalunya favors rural voters in representation, and rural voters are more pro-independence than urban ones, largely in Barcelona. A poll taken by the government as recently as July showed 48% opposed to independence with only 41% for it. The pro group has at times beaten the anti group in polls, but in fact the pro group has never gotten above 50% in a poll, although they got 90% in this illegal referendum, which will be taken very seriously by world history, given that this stupid judge sent the national police in to beat people up.
The problem runs deeper. While indeed the province has autonomy in education and language, many charge that the school system has been taken over by nationalists who are trying to eradicate the Spanish language. Still 60% of the provincial population speaks Spanish as their mother tongue. This is a very close call, but the critics charge that the nationalists have been going out of their way to suppress their opponents in an undemocratic way. I can dismiss the national Spanish courts and the EU supporting them, but in fact members of the current government quit in protest over the way the move to push this referendum was done in the Catalan parliament itself was handled. They may accuse their opponents of being remnant Franco Phalangist-fascists, but their own tactics resemble that ideology more than those of their critics, just as the Lombard League in Italy goes neo-fascist.
My own personal observations on this really involve a visit I made in 1973 to Barcelona. I note I was taken to Spain (not Barcelona to my knowledge) in 1954 at Easter time by my parents when I was six and unaware of politics. I remember getting upset when my sister and I were left in our car in some small town while my parents got something, and a crowd of people came and banged on our car begging for money. I suggested to my father that we give them some, but he said that there were too many, and if we gave any to some, others would demand more than we had. Later I saw penitents looking like KKK guys in Toledo. We bought a figure looking like them and many decades later my grandson Charlie was frightened when young when shown this odd figure. Then in Granada, we saw people living in caves. Really.
Anyway, getting back to well-off Barcelona in 1973, Franco was in charge then, and indeed he totally suppressed all languages other than Castellano Spanish and maintained a strongly centralized control. Every sign was in Spanish in Barcelona, but I heard people speaking Catalan, which I figured out because the words they used were neither French nor Spanish, although more like French (and later I figured out more like Italian really). They did so obviously when no authorities were around, and it was clearly an act of resistance against the dictatorship. I was sympathetic.
Since then they have achieved sufficient autonomy that their language is everywhere and now taught in schools. Any claim by Catalunyan nationalists that their language is being suppressed is just a pile of horseshit, to use a technical term. I have seen now the opposite when visiting Barcelona: people being humiliated and rebuked for speaking Spanish in public. This disgusts me just as much as the 1973 suppression of Catalan under Franco disgusted me. I have no sympathy for these rich and spoiled crypto-fascists with their arrogant public behavior toward anybody speaking Spanish openly, although I oppose any effort by anybody in the central government to engage in violence to suppress the obviously burgeoning and almost certain to succeed independence movement.
A final note is that my old friend Paul de Grauwe, one of the fathers of the euro, has posted (sorry, too lazy to link) comparing the Catalunyan independence movement to the Brexit movement, and implicitly to the Trump movement in the US. Obviously there are differences, but the similarities are striking. Among them are outright delusions, particularly for the Catalalunyans that once they are independent they will be in the EU and have great economic outcomes. Ooops, but no. They will have one economic gain: not sending tax monies to the poorer regions of Spain. Beyond this, they will lose. They will not be in the EU, and will not be let in soon and will not get those benefits, just like the UK, where lots of idiots voting for Brexit thought they were going to get their cake and eat it too, but are now gradually waking up to the fact that this will not happen. They are going to pay and pay big time. I shall not draw out in any detail the comparison with the US situation.
Anyway, I think this is tragic, and I think it is partly a response to the election of Trump in the US: irresponsible lying creeps feel free to override normal procedures and rules to seize power and push forward racist nationalist agendas all over the place. This one is less likely to end up in outright war and people dying, aside from the several hundred injured in this referendum vote, but I fear that there will be nothing at all good coming from this, nothing, and quite a bit bad, both for those in Catalunya/Catalonia, as well as the rest of Spain, and possibly in the rest of Europe and the world.