In the anime, the main regions' Pokémon Leagues (Kanto, Johto, etc.) all have the same structure:
- Acquire the eight regional Gym Badges
- Travel to a stadium to compete in the regional League Conference
- Win the Conference, and earn the right to challenge the Elite Four and Champion
- Defeat the Elite Four and Champion, and become the new Champion.
The exception is Alola, which has never held a Pokémon League before and thus had no Elite Four/Champion. By winning its Conference, Ash therefore became the new Champion automatically.
The Orange League was a "filler arc", to stall for time until Pokémon Gold and Silver were released and they could begin adapting it. As such, it was much shorter:
- Acquire the four regional Gym Badges (most of which are acquired through challenges other than Pokémon battles)
- Beat the Supreme Gym Leader
- Become a Champion, but not the Champion (so you don't become the new Supreme Gym Leader and have to face challengers like the other leagues; it's purely an honorary title).
The Orange Islands arc is canon, as far as I can tell. But because the Orange League is
a) so much shorter, including omitting the Conference that was Ash's main stumbling block, and
b) not in a region from the games, it's generally considered to not count.