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In light of recent events, here's OpenStreetMap editors discussing naming of the Gulf of Mexico
(community.openstreetmap.org)
This is a most excellent place for technology news and articles.
Because the United States Board on Geographic Names now says so, as demanded by Trump by Order No. 3423. They are the government body that defines what places are officially called.
But unless you are a US federal employee, as Nick Fury once said, "I recognize the council has made a decision, but given that it's a stupid-ass decision, I've elected to ignore it."