Considering that his Hydreigon is lower levelled then before, lore kicks in and creates the possible idea that he must have misused (and possibly abused) it for losing to the protagonists two years before.
I just watched a video called “Pokemon Riskiest Game” I highly recommend it giving that a watch. What he said is that Ghetsis’ Hydreigon hates him and could’ve left him at any time however due to the technology of Pokeballs, Hydreigon is forever bound to be abused by Ghetsis which gave a whole new perspective about Pokeballs. The very thing that we, the trainers, use to make bonds with our Pokemon
@@Komodo772 Yeah, in Black 2 & White 2, Ghetsis himself uses a cane to force Kyurem into battling you before he proceeds to battle you with his full team of six.