While it is more commonly known as the Indigo League, the names "Indigo League" and "Kanto League" are simply synonymous
Depending on the context, the Indigo League can also be called the Kanto League or simply the Pokémon League
However, after doing some research on Bulbapedia, I found the reasoning to why "Kanto League" is known as the "Indigo League"
In the Generation I games, the final area of the game is called "Indigo Plateau" which lead to many players assuming the Pokémon League for this region was called the "Indigo League". The reason why the name "Kanto League" wasn’t considered was because Kanto wasn’t commonly known at the time. “Kanto” was only ever mentioned once in the Generation I games, and not mentioned outside of Japan until Generation II games.
Until the release of Pokémon Gold and Silver, Kanto's name was seen only once in the Japanese Generation I games: upon viewing the Town Map at Blue's house. It was never mentioned in the Generation I games outside of Japan, and thus many fans assumed the region was called "Indigo", based on the name of the Indigo Plateau. By the time the Generation III remakes, Pokémon FireRed and LeafGreen, were released, Kanto's name was firmly established and appeared regularly in-game