Because Gulf of Mexico is a long standing and universally recognized name. It does imply Mexican ownership and there was no need to change it. Changing its name only makes us look petty.
So what? There is a new boss in town and millions of Americans love the change. It's not stupid as you stated.
And the only reason you stated that is you don't like Trump. We both know that. And the overall majority of those who do not like it, are American hating leftist, Communist and assorted posers.
Looks like you're in good company Petros...