Because the Champagne administrative province is in France.
A light wine, of several kinds, originally made in the province of Champagne, in France.
Champagne is one french province that is known for its sparkling white wines.
Reims was the capital of the Champagne province in the olden days. But in modern France, the capital of the Champagne region is the city of Chalons en Champagne
Picardie claims it with some justification, but so does Champagne, where the wine comes from, and the old province of La Bresse, where the chickens come from.
Britain is the world's largest importer of champagne.
France
The word 'champagne' comes from a late Latin word whose use dates back to the third to sixth centuries. That word, 'campania', refers to level, open country. It in turn traces back to the ancient, classical Latin name for the present-day province of the same name in modern Italy. The capital city of Campania is Naples. And the term 'campania' as 'level, open country' is descriptive of both the regions of Campania in southern Italy and of the champagne producing region of France.
France
Champagne is a protected name for only made in France from the Area Champagne in northern France. The other name outsdide the Champagne in France is Crémant. In Germany and Switzerland the name ist "Sekt". In common usage, champagne refers to any sparkling wine from any country.
Champagne region of France, although Mumm owns some vineyards in USA. Legally that wine cannot be termed "champagne".
The US is not a province. It is a country.