Okay so I know the reason: they didn’t put her in the Ash anime because they wanted to emulate Indigo League. Indigo League had:
I kind of agree. Just as the games from the first of the 5th generation came with the proposal of a reinterpretation and perhaps even a reimagining of the first games in the franchise and the Pokémon themselves, the anime, equally, sought, based on the original animated series, to reinvent itself once again. I don't think the producers wanted exactly emulate the Indigo League, but rather take some inspiration from the firsts seasons and structure the series in a new way.
However, I have the impression that they failed in this, which is why they returned with the previous approach in the following series, X/Y.
It's not really weird, XY doesn't have any other female characters to use and Bonnie was a little kid. Shauna was a rival in the games so she wasn't considered for the cast.
Personally, I believe the producers demonstrated with Korrina in X/Y that if they wanted to they could have made her a protagonist, like Misty in the original series.They even had her grandfather to replace her at the gym, while her plot could revolve around Aura and Mega-Evolution, stealing the mini plot that was dedicated to Allan.
For AG it wasn't even that May was the first girl, they already used Kris/Marina in the Raikou special a year before AG started in 2001. I always thought May was loosely based off her character since Marina was an idol trainer and then became a coordinator after.
Well, before the Advanced series, Marina only appeared in a single special episode, or mini movie, related to the nostalgic Pokémon Chronicles series.
She wasn't a coordinator on that special, and she wasn't even an Idol. Instead, she just tells Jimmy her desire to participate in Show Battles with her Pokémon, and is mocked by him as wanting to be a Star, instead of dedicating herself to serious training.
Perhaps, in Marina, we can have a mere indication of the producers' desire to give more space and a maybe a more central role, but different from that given to the male protagonist, to the female characters in the anime.
Now saying that she preceded May, and that May is loosely based on her is a very farfetched assumption, especially when Marina herself added so little to the animated series. I don't see how the use of Marina had any implications for what followed in the Advanced generation.
