Because there are several other languages (Galician, Aragonese, Asturianu, Estremeñu, etc) that are just as Spanish as Castilian is.
Did you notice that English is called English (from England) and not British?
Quite logical IMHO, as in the UK several other languages are spoken (Welsh, Irish, Cornish, etc)
In the last election, the parties with the most votes were PSC (23,04%), ERC(21,3%) and JxCat (20,04%). PSC is unionist, where ERC and JxCat are separatist. Ciutadans (radically unionist) merely got 5,57% of the votes.
That's simply not true: Spanish Constitution do allow autonomous communities to organize referenda.
As a matter of fact, quite a few have been held:
* Basque Country, 1979-10-25: Referéndum sobre el proyecto de Estatuto de Autonomía para el País Vasco
* Catalonia, 1979-10-25: Referéndum sobre el Estatuto de Autonomía de Cataluña
* Andalusia, 1980-02-28: Referéndum sobre la iniciativa del proceso autonómico de Andalucía
* Galicia, 1980-12-21: Referendum sobre el Estatuto de Autonomía de Galicia
* Andalusia, 1981-10-20: Referéndum sobre el Estatuto de Autonomía de Andalucía
* Catalonia, 2006-06-18: Referéndum sobre la reforma del Estatuto de Autonomía de Cataluña
* Andalusia, 2007-02-18: Referéndum sobre la reforma del Estatuto de Autonomía de Andalucía