It's clearly obvious that the game and the show are completely different. The reason that Victory Road is game exclusive is that it makes the game more challenging for the player. Also the fact of the games series having a individual vibe going on, everything done is by the player's actions. One the other hand, the anime always had Ash having a group consisting of himself, Pikachu, a girl, and a cook traveling together. Having that group traveling a giant maze-like cave every time Ash was ready to face the E4 would be quite odd. A festival would negate the awkwardness of traveling through a giant maze-like cave (at least TV wise) and act as a filler and transition for the next season.
I hope I helped and made things a bit more clear.