I remember that this fantastic game had spanish language support.

¿Why is not spanish included?

Indeed, in its launch it was published in Spanish, and it seems that in gog they still do not know that Spanish is one of the main languages of the world.

I'm sure they know of the Spanish language's prevalence in the world.

The issue is likely more akin to why there are localisations missing from other titles as well. Which sometimes is that the rights for one of the localised versions, like in this case the Spanish one are not held by the publisher, but, for whatever reason, are held by the studio which did the localisation back then.
And if that studio, again for whatever reason, has not given the thumbs up for their localised version to be used, there's nothing the publisher or GoG can do about it.