Localization is not trivial, especially for a heavily story-focused game like FFXIV. It’s not just translating the text and names. Characters and content may get changed to fit the culture and/or laws of the country that it’s released in. And a support team needs to be established for the language/locale. (Probably a set of forums too, looking at how the forums are structured here.)

Games are usually tested and supported using the official language of the country that it’s released in. The official language in the United States is (American) English. It is expected that the players within the US (and Canada) are fluent in English, even if it’s not their first language. So they likely do not see much benefit to adding Spanish support.