They took too much metal out.
No, they didn't put enough in.
The name of the game is to make the wing as sturdy as the certifying regulation requires it while at the same time keeping the weight as low as possible.
Putting too much metal in and making it too strong adds unnecessary weight that bears on the overall performance and operating cost of the aircraft.
Reinforcements were subsequently made and the wing was certified.
And that was the A380, by the way, not the A350.