Here's why:
* Asgard technology is generally more advanced: Asgard technology is considered superior to Ancient technology, and their shields are no exception. While Ancient drone weapons are powerful, they are unlikely to pose a significant threat to Asgard shields.
* Ancient drones are primarily used against Goa'uld: The Ancient drones are primarily designed to combat Goa'uld forces, not to engage Asgard ships.
* There are no instances of Asgard ships being targeted by Ancient drones: There are no known instances in Stargate where Asgard ships are targeted by Ancient drones, let alone instances where the shields fail to resist the attack.
It's possible that the showrunners chose not to depict such a scenario for various reasons:
* Maintaining Asgard superiority: Showing Asgard shields being breached by Ancient drones would undermine their technological prowess.
* Avoiding redundancy: The show already depicts Asgard shields resisting other powerful weapons, such as Goa'uld weapons and Replicator weaponry.
Therefore, while it's conceivable that Ancient drone weapons could pose some threat to Asgard shields, it's not something that's been shown in the series.