The answer from SJuan76 is very well pointed, but not 100% accurate. The economic prevalence of Catalonia was true for decades, and encouraged first by dictator Franco and later by the nationalist parties threats that got all kind of economic privileges from the national Spanish democratic governments in order to maintain themselves (both sides, the Catalan nationalist regional governments and the central national government) in power.
A kind of quid pro quo and everybody happy (everybody means the politicians...): "I overlook elsewhere and I pretend not seeing your corruption in Catalonia if you support me to get majority and rule in the national central government, and you also look elsewhere regarding my own dirty clothes and corruption and other Spanish regions".
But in the last decade the Catalan economy has been falling slowly but steadily while other regional economies rose and grew strongly, namely the economy of the Madrid metropolis and its surrounding region. Today the economy of Madrid (one province) is by far stronger than Catalan economy (four provinces).
Besides, Madrid continues going up and Catalonia down. And this "folly towards the independence race" -Financial Times's words of today November 12th, 2015, not mine- is very seriously damaging the regional economy and notably the foreign investment in Barcelona and Catalonia.
Many foreign multinationals are discreetly warning that if this goes further they will move sites and investments to Madrid or other Spanish regions; not just foreign companies, a few thousand Catalan companies have moved out of Catalonia only in the last two years. Moreover two of the biggest Spanish banks (CaixaBank and Banco de Sabadell) are Catalan (not only because of their sites and origin, but they are as well proudly Catalan and it's in their "blood", culture and statutory foundation), anyhow they have most of their business in the rest of Spain and have changed their statutes and announced that they are preparing to move their sites from Barcelona province to Madrid. That means something.
But the main point that you overlooked is the real reason of this crazy race to nowhere. The nationalist politicians and the elite lobby very deeply associated to them, who have ruled Catalania's regional government and economy for the last thirty years are today very seriously in trouble (probably they could finish in jail) and being investigated by several judges because of continuous fraud to the law and taxes fraud. Everyone who wanted to make business or just establish their Spanish subsidiary in Catalonia had to pay a toll of minimum 3% to the nationalist party in the regional government and the ruler (Jordi Pujol) and all his partners (including today's last regional president and leader of this "folly", Arturo Mas). This includes all businesses associated to the Barcelona Olympics in 1992.
You can ask to the last publicized company who paid such ransom just to open a factory in Catalonia and get J.Pujol to inaugurate it (and probably other future privileges still sub iudice): the pharmaceutical Ciba-Geigy.
The numbers that the Justice is discovering that the nationalist leaders (and the nationalist party, that was officially involved technically in the reception of these illegal commissions as well) defrauded to all Catalans and Spaniards and evaded to fiscal paradises are reaching today (November 12th, 2015) 6,500 millions euros.
And these numbers being publicized by the Press are growing steeply each day, because we know today only what was evaded to half of the Andorra banks and from just a few of all the other tax-evading paradises that they used. Other banks in those paradises have still to send the official statements of the money evaded during these 30 years. And besides, all of this is shown to be quite hard and long to prove and know, because they have moved the money from bank to bank, from country to country, and most of the economy and social Catalan ruling elite is seriously involved in these commissions and evasions businesses, and therefore not very favourable to denounce it.
Curiously they are those promoting and supporting this independence, for the most part.
Another point. These last years the Spanish citizenship has awaken and wants to change the actual political statu quo, all over Spain, including Madrid, Catalonia, regional governments, national government, town halls, etc., and in all political sensitivities. Don't forget that the "Occupy Wall Street" movement "was born in Spain" with the 15-M movement, root of today's new rising radical party "Podemos" (We can), with similar demands (at least in the origins) to the Greek Syriza. At the same time, another new party, "Ciudadanos" (Citizens), is growing crazily in the expectations in just one year, from zero -they didn't exist before- to the second position both in the national Parliament and in the last ejections to, a few days ago, to the Catalan Parliament.
The traditional parties, from left to right, in every region, are losing elections and people's support in favor of these newly born parties day by day. The hard economical crisis, affecting especially the middle and low classes, the corruption discovered day after day in all regions and all ruling parties, and the fact that almost nobody goes to jail, gets fined, returns the money, or simply just resigns. Nobody... In Spain you can steal, lie openly, and promise the moon, and people will still vote you... Until now!!!.
And the traditional parties don't know how to react to regain power and fight back these new young clean parties that have the endeavour to regenerate the politics in Spain and who don't use anymore the usual labels of left, right, and they speak with a new speech and and a totally new discourse.
To this every political party chooses to react in a different way to try to keep in power. Catalan nationalists chose to become more and more radicals, trying to hide their corruption and economic failure behind the independence flag. Simply.
To conclude, this crazy race forward to the vacuum outside of Europe (citing the Financial Times) has a main hidden reason: to avoid going to jail. If Catalonia becomes another country, Spanish rules will not apply (and European ones as well), and they can make new rules to just keep in power no matter what (by the way, avoiding having to pay back what they stole or having to go to jail).
They have "extremed" and exacerbated the situation within the region during these last years, so that they managed to have now a situation of total confrontation of Catalan citizens in two opposing sides in Catalonia: half of the Catalans are for the independence (49%), half for staying in Spain (like in the last 500 years) and comply with the Constitution and the Spanish and European rules (51% of the votes in the last regional elections few weeks ago).
If because of an allegedly idealism (that hides escaping corruption behind idealistic flags), you decide to break every rule to achieve your goals (they just broke yesterday the own laws of their own Catalan Parliament, because it didn't fit their interests), what kind of legitimacy can they have when they rule their own country? Who will trust them? They have technically the simple majority in the new Catalan Parliament, even though most of electors vied against them (49% vs. 51% of votes). Blame the stupid
D'Hont electoral laws applied in Spain, that nobody changes because it favours the biggest parties, no matter if they get less total votes, however it gets the traditional parties more Parliament seats with less votes. (By the way, both new parties, Podemos and Ciudadanos, have promised to change this unfair electoral law if they win the next Spanish elections, by the end of this year, December 20th.)
Big companies and investment trusts won't trust them, for sure. They are already voting and reacting...
Beyond that, take your own conclusions.
P.S.: I am not Catalan nor from Madrid. I am from another Spanish region.