“Our focus is bringing MCC and Reach to PC,” 343’s Brian Jarrard wrote on Reddit during an AMA. “There are no current plans to bring over Halo 5.”
Just because that’s the plan at the moment, it doesn’t mean things will remain that way. Remember that Halo: The Master Chief Collection originally launched with Halo, Halo 2, Halo 3 and Halo 4 and later added Halo 3: ODST. A recent announcement confirmed that Halo: Reach will be added to the mix later this year.
Halo 5: Guardians is currently available on Xbox One, while Halo: The Master Chief Collection is currently available on Xbox One and heads to Windows PC via Windows 10 and Steam later this year.