Due to the lack of playtime, Riot Games might not be bringing back unique game modes like Odyssey, Star Guardian, or Blood Moon any time soon.
Many League of Legends fans have been complaining about why Riot Games is not bringing back some unique game modes like Odyssey or Star Guardian. In response to the complaints, in the latest Dev post, Riot Games explained why they are doing this and why they might never bring them back.

Both the Odyssey: Extraction and Star Guardian: Invasion were PvE-style game modes that had a unique play style. Considering the concepts, these events should be fun, but if we take a look at the chart provided by Riot Games, we can clearly see how quickly the play hours of these modes dropped after just a few days.
If you played Odyssey: Extraction in 2018, you might remember how frustrating it would sometimes get. There were countless reports on League of Legends communities on how many players just played these modes to complete their missions and never played them again after completing or never played them at all.
“Modes like this put a really unique twist on League’s core gameplay, but they didn’t hold your attention for long,” Riot Games said. “In other words, players were excited to try the modes but would quickly return to Summoner’s Rift after the novelty wore off.”
Game modes like ARURF, on the other hand, had a massive spike at the beginning, but ultimately, play hours also dropped significantly. ARAM and TFT are the only game modes that still manage to hold a consistent game hour.
“We want to make sure that the amount of time we spend in development is worth the amount of time y’all get enjoying it,” Riot Games added.
But Riot Games did bring back some popular game modes like Pick URF and One for All in recent patches.
Based on today’s dev post, it is clear that Riot Games is not planning to reinstate those events any time soon, but, in the meantime, they will be focusing on improving things like the Event Pass system and mission variety.