Which has to do with the original game not being designed around HD.
A lot of Xbox and Halo fans rejoiced when they learned that Halo 5: Guardians will be getting an update for Microsoft’s upcoming Xbox One X console. Today, Frank O’Connor of 343 Industries detailed the specifics of that update on NeoGAF.
O’Connor noted that while the update will include 4K textures, assets, and resolution, support for HDR might be limited- which comes down to the way Halo 5 is built, and what HDR is. Since HDR requires specific lighting models, everything needs to be retouched or balanced around it- which is fine when the game is built around it, but hard to implement into an existing, older titles.
That said, O’Connor was quick to note this will not be a problem for Halo 6 or future titles- since they will all be built around the HDR spec and expectation to begin with.
Halo 5 is available exclusively on the Xbox One; the Xbox One X will be releasing on November 7.