Yeah you're right.
So far, this is the only episode where Axew uses Outrage.
That's from Bulbapedia's Page on Episode 42 BW
As to why?
There's been a discussion on this before on SerebiiForums
It was so Iris could move on, nothing more.
Because it is strictly a finishing move, meaning that Kibago can't use it unless it absolutely has to. Seeing as it falls asleep right after using it you can see why Iris might not be commanding him to use it can't u. Although I could see a training g episode where Kibago gets over falling asleep afterwards lol.
I think the reason Axew hasn't used Outrage is simply because it can't.
It's called lazyness. From beginning, Axew has been nothing but a punching bag always being kidnapped. Now it learns Outrage. So now what? Axew isn't going to ever use it again until there is a Langley or another near death episode? Why can't Axew train to get better at it?
Originally Axew used Outrage in an extremely high pressure situation. It was the semi-finals. It's trainer Iris had a lot riding on this match and it was all up to him. Axew dosen't want to let anyone down. It had received a pretty thorough beat down from Gollet thus far and was standing on it's last legs.
Saying "The writers forgot" would be a bit of an overstatement
I seems popular opinion is that they intentionally neglected it for reasons we can only speculate - whether it was just a one-off move for the purposes of the single episode, or it was laziness etc.