>>48078807Most likely, they didn't want to give Ash an older travel companion, so their solution was to use the character that was closest to his age.
Which in the end did not matter, other characters like Mallow adjusted to Ash's age without much problem, they also used a teenage Bianca from BW1 and a young adult Cheren from BW2, so having a 10 year old Hilda seemed viable to me.
They just wasted the best girl in the franchise and it was only years later that they realized it, but it is too late, there is no way to include her without doing it forcibly.
If she is lucky, they will give her a one-second cameo at the close of the anime.