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)
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)