Since Hilda isn't in BW2, that means Hilda only had what...roughly 2 years to be the star female in the game? Unlike May and Dawn, who were basically the main females for their entire generations, (aside from FRLG and HGSS), Hilda is only considered the main female for half of Gen 5 in the games.
I think if the writers knew Hilda wasn't going to be the main female for both gens, they'd have to replace her when BW2 came out, essentially bringing in 2 new girls for Gen V. Instead by going with Iris, they don't have to bother with the two game females this gen and they can be ignored.
I think if the writers knew Hilda wasn't going to be the main female for both gens, they'd have to replace her when BW2 came out, essentially bringing in 2 new girls for Gen V. Instead by going with Iris, they don't have to bother with the two game females this gen and they can be ignored.