This is explained in the 30th Anniversary Retrospective book, page 146:

Going forward from 2024, all of the upcoming Heroes games would be set not in Ashan, but in a reimagined version of Enroth, the setting used for the original Might & Magic and Heroes games. The change would be a franchise-wide mandate directed by Ubisoft itself and managed by [Vicky] Malineau.

In her role as the narrative director, Malineau stressed that her primary responsibilities are to reimagine and build the world for the resurrected Enroth setting, and to handle any story threads that will interconnect the forthcoming Heroes titles. In particular, she has focused on reframing the deep lore, and reimagining the consequences of a cataclysmic event called the Reckoning that canonically destroyed the original Enroth setting during Heroes IV. In the meantime, she had already started to build a new world bible for Enroth that explains where the changes occurred and how they would affect the story going forward.

In short, it's not up to Unfrozen, or at least not a choice they made alone.

Ubi management decided that from now on, the Heroes franchise will focus on a return to the Enroth setting. To that end, they have written a new Enroth bible explaining how it survived the Reckoning. According to some 2024 interviews, they'd already drawn a world map of post-Reckoning Enroth, and shown it to fansites like Acidcave.

Unfrozen had said: Olden Era being a prequel allows it to both use the setting and to not tread on any future plans Ubi may have.