>>52491127Because she knew Ash was expecting it, which we also knew because we all watched the same battle. Ash had prepared for Mega Garchomp, but no one knew about Dynamax Togekiss, and she took advantage of its Dynamax moves to gain the upper hand over Mega Lucario.
In the end however Mega Lucario’s tenacity was just too overwhelming. And Ash, being the smartass that he is, was hiding Reversal the whole time, so Cynthia would’ve lost regardless if she used Mega Garchomp or not.