Because it never has been translated in Spanish.

Translation involves a lot more than just putting something into a software and spitting it out, it involves a whole team of staff and voice actors. SE has always translated into the listed languages and so they have the staff and resources ready to go. Maybe in the future they will add support in Spanish for a game, but probably not this one, sorry.

There is a significant population of people who would enjoy a Spanish translation of the text at least. Maybe a good fan project would be to translate things to Spanish.