Because for Kalos, it makes no sense to through a large chunk of Kalos with only 5 Pokemon.He doesn't need a full roster for the gyms, and it seems likely that he will use at least a few old Pokémon in the league. Between Goodra possibly returning and Charizard/Sceptile Mega Evolving, I don't see why Ash needs a full Kalos team. One more capture should suffice.I could understand it if Ash had a full team of Pokémon, but Ash has only five Pokémon thus far. They're running out of options for possible teammates for Ash, so I don't think releasing one this saga is going to do him any favors. Unless Ash brings back one of his old Pokémon that happens to be in one of the Kalos region's Pokédexes (which I highly doubt), they might keep Ash from having a full team of six this saga. That's not such a good thing for the writers to do.
Not counting Goodra, Ash's team is woefully inadequate. It seems like the only Pokemon that ever seems to be enough is Pikachu.