[cronopios]

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)

[elnatro]

That’s nonsense in Italy and Germany there are other languages/dialects too and nobody changes the name to the main common language.

The case of English is different as the language was mainly used una part of the country, while Spanish rapidly extended to all regions.

[ivanbakel]

Italy and Germany were both unified much more recently than Spain, and by popular nationalist movements. The selection of their national dialects was basically artificial, since Tuscan Italian and Hochdeutsch were not widely spoken in either country.

The status of languages like Castillan is much more controversial, because they are only the national dialect by conquest. (French is in a similar position, though it cleverly shares the name with the country it is the national language of.) It doesn't help that the French and Spanish governments have historically suppressed "dialects" in favour of the national language in an immoral and shameful way.

>The case of English is different as the language was mainly used una part of the country

English is the most used language throughout the UK. There are almost no non-English-speaking communities left in Wales or Scotland, and language (much like everything else to do with the English) is a contentious political issue in Northern Ireland.

However, it would be grossly inappropriate to start calling this language "British", even when just referring to how it is spoken in the UK - because of many of the same historical features as Castillan and French.

[darkwater]

> That’s nonsense in Italy and Germany there are other languages/dialects too and nobody changes the name to the main common language.

Because Italy is still in its nationalist infancy, being united as big, single, 19th century cool nation for a little more than 150 years. But many of the "dialects" spoken in Italy are actually different languages with different vocabulary, grammar rules etc.

[cronopios]

So, according to you, the Spanish constitution is nonsense:

Article 3

1. Castilian is the official Spanish language of the State. All Spaniards have the duty to know it and the right to use it.

2. The other Spanish languages shall also be official in the respective Autonomous Communities in accordance with their Statutes.

3. The wealth of the different linguistic modalities of Spain is a cultural heritage that shall be the object of special respect and protection.

[elnatro]

No, officially in Spain the language is Castilian. However, according to the RAE, it’s also called Spanish. Indeed the RAE recommends the use of “español” (Spanish) [1].

[1] https://www.rae.es/dpd/espa%C3%B1ol