In an interview with Famitsu, Kojima explained why he feels the Ground Zeroes prologue for Metal Gear Solid V is needed.
According to Kojima, the prologue serves two goals: laying down the foundation for Snake’s story in this game as well as the overall theme of revenge, and getting the player used to the open world structure.
“GZ takes place in 1975, a year after Peace Walker, and the main game takes place in 1984, nine years later. Something bad happens at the end of GZ, and then you continue into the main game.
Unless you play GZ, you won’t understand why retaliation is one of MGS5′s main themes. It really is the ‘ground zero’ of Snake’s story.”
Kojima also mentioned some of the limitations put in place in the prologue.
“We’ll limit the amount available to you at first and set a static time and weather pattern. So you’ll get to play this open-world, highly free MGS in those conditions.”
So, time won’t pass and the weather won’t change dynamically in Ground Zeroes, as opposed to The Phantom Pain.