For the British, at least, it was seen as a victory because it forced the French fleet back into harbor and allowed them to blockade it, even though the French grain convoy got through. The French also claimed victory, and strategically they did deliver the grain and quell fears of a famine, but operationally it was a defeat for them.

If you're asking why the battle is named as such, it's mostly because it was unique in the Age of Sail, not being fought near land and thus not named after a landmark.

If you're asking why it's called "Glorious" even though the captains on the British side did not all cover themselves in glory, it's mostly due to the Admiralty presenting it as a great victory to the public, and the adjective attaching itself to the battle's date.