>>32360578Because they never did something big like Serena till now. Pokeshipping was dropped very early into Misty's run and they never promoted another Ash ship again. There's no reason for them to have another girl crushing on Ash like Serena again, not only would it be redundant, but nobody will believe a ship for a new girl again.
Also Tomioka co-wrote the last XY episode with Yajima. Tomioka has been with the anime since Kanto episode 3 (he wrote Ash captures Caterpie) and has been with the show every series. If he's around he'll most likely continue to leave Serena as an endgame.
