Locking down a singular narrative on why this is has been tough, to say the least. Some of the people I've spoken to lay blame at former game director Chris Lee, while others lay blame at studio head Bonnie Ross. The one consistent area of irritation falls on the Slipspace Engine, however, which has become somewhat notorious in commentary surrounding Halo Infinite for its difficult implementation.
I know from information and conversations that I've had about Tatanka that the game was supposed to incorporate Halo Infinite, and Certain Affinity received Halo Infinite's source code to build out its implementation. We know from leaks in the Halo API some fragments about how Tatanka is meant to play, although if the game is indeed switching engine as rumored, it's essentially anyone's guess just how much of the original plan will be retained.
Instead of representing the continuation of Halo Infinite, it's possible that Tatanka could eventually grow to represent the next phase of Halo itself as a completely separate, standalone experience. On a different engine, it may scupper Microsoft's original plan to incorporate the Forge mapping tools, but they could still bring across Halo Infinite's cosmetics potentially by leveraging Halo's social APIs.