https://www.gamespot.com/articles/why-overwatchs-pve-modes-are-only-available-for-a-/1100-6458124/

Overwatch's next major PvE event is now live. As a part of the Overwatch Archives series, Retribution takes cues from last year's Uprising event, pitting four players against an onslaught of AI-controlled enemies in a narrative-focused mission. Detailing one of the last missions conducted by Blackwatch, Retribution is set eight years before the events of the main game, focusing on a doomed operation with McCree, Genji, Moira, and Gabriel Reyes--who would eventually become Reaper.

"Right now the plan is to not have [Retribution] available after [the timed event]," said Overwatch director Jeff Kaplan during a press Q&A. "We are constantly revisiting those types of decisions with some stuff that we've done--capture the flag is a good example. In our previous event for Year of the Dog, where we introduced a brand-new map for it, we figured it was evergreen and kept it into the system. So right now the plan is to only have it available for the event for about three weeks."