It wasn't laziness or lack of effort, forgetting cultivation, but a neglect of the heart's state, forgetting anger.
Because things have been going so smoothly lately, their strength improved too fast, the people around them were far too kind, both of them actually somewhat forgot their anger towards this world that has hurt them.
Also, slightly... dulled.
This is not surprising.
Since An Jing became a Martial Artist, even though he wasn't a Grandmaster, just an ordinary Inner Strength Martial Master, he rarely encountered anything truly ugly, something that could make him angry once more, not to mention he now possesses the power of the Divine Repository, half a foot into the True Master Realm.
Perhaps it exists. But those things are so grand, so majestic, that even the ugliness of the Taiming Sect extinguishing all beings grants a kind of overwhelming shock.