Because Spanish is the Americas' "language #1", and the second most commonly spoken language within the United States by a wide margin. It's courteous to acknowledge the largest minority group: in San Francisco, Muni's announcements are also made in Cantonese (the subway in Canton only uses Mandarin and English); Chicago long ago had German-language public schools, and today the CTA prints up official announcements in Polish and Spanish.

You also didn't respond to my point about cross-fleeting. Having "bilingual" domestic aircraft is a practical matter.