Why is the GP7200-powered A380 fleet not grounded?
The one that decided to ground the fleet was Qantas. Qantas was having a new airplane model and potentially made a trade-off between having a not known design flaw and keep flying and the risk. At that time the A380 entered in service three years before and around 20 aircraft were in service with a mix of engines.

Today, there are 200 A/C in service for 10 years. The possibility of having a design flaw or an unknown problem is really low.

It was not a airworthiness directive as the directive was created 2 months after in January 2011. Qantas starting to get back in service the airplanes on 27th November (incident was on 4th Novemeber)