The reason, according to Bungie, is due to the short amount of time the developer had to respond to the problem and adding the fix to the scheduled December 12 Destiny 2 update. Bungie explained on Twitter:

This past weekend (‘Laser Tag’ weekend, as referenced by Bungie), the Prometheus Lens ran amuck in the Crucible and Trials of the Nine because as a fast fix, Bungie purposefully placed Prometheus Lens in Xur’s inventory making the OP weapon available to virtually every player in Destiny 2. Some players loved the move, even preferring the fast, Call of Duty-style time-to-kill in Destiny 2, while others surely avoided the Crucible outright.

While it’s encouraging that Bungie is responding quickly and openly in this case (especially in comparison to how slow and silent the developer was shortly after the game’s launch), some players are sure to be disappointed that Prometheus Lens will be so weak even in PvE where it wasn’t a problem.

The second December update that will nerf the Prometheus Lens is expected to go live in the morning PST of Tuesday, December 12.

Destiny 2 is available now on PC, PS4, and Xbox One.

Source: Bungie