"If we just threw you into the main open-world game, we figured that veteran MGS fans would get confused," series creator Hideo Kojima explained.
As an example, he cited Snake being fatigued after a mission, and needing a short break. He said Ground Zeroes will have some of those dynamic elements, but will keep others fixed so that players can get into a few basic concepts before jumping into the full game. "So to keep people from thinking 'This isn't MGS!', 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," Kojima told Japanese magazine Famitsu (via Polygon).
Once again, Kojima reiterated how the two games relate to each other in terms of story. "GZ takes place in 1975, a year after Peace Walker, and the main game takes place in 1984, nine years later," he said. "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."