Because she never was that strong, sure she was stronger than Mallow but look who that’s comparing her to. Once she was up a bigger force than herself; she got laid out.
I think that was the recurring issue with them, they weren't challenged often. They were kept very complacent without much pushing them in battling or whatever else they put their attention to. They accepted to showcasing their limit in battling, but at the eleventh hour in the series where they can't really play off of it in a later arc or anything. Their roles in cooking and bubble making weren't really offered new challenges or steps either.
I think Sandy and Shaymin suffer in that regard. The other secondaries among the group represented a new step for the trainers to adjust to, a hole in their agenda their signature couldn't currently fix, but the two don't really have any substance to play on for Lana and Mallow.