According to Metal Gear Solid director, Hideo Kojima, releasing Metal Gear Solid 5: Ground Zeroes before Metal Gear Solid 5: The Phantom Pain was not his original plan.
Apparently, as stated by IGN, the famed director wanted to release the upcoming introduction, Ground Zeroes, at the same time as the main game, The Phantom Pain. The reason for changing this plan is all about the reportedly enormous size of The Phantom Pain.
Due to the main game’s size, it was apparent to Kojima a while back that the game would never have been ready for the launch of the Playstation 4 in Japan. Knowing that Japanese buyers would want a AAA title at launch, it was decided to split up the game’s introduction into what is now known as Ground Zeroes. This way, consumers could have a taste of what the MGS crew was working on.
This basically confirms what I’ve believed all along which is Kojima is giving us Ground Zeroes as fan service; a chance to enjoy the beginning of what will be an amazing, enormous title when it eventually released.
In related Metal Gear Solid 5 news, it’s been stated that The Phantom Pain is so large that Kojima is actually worried that many gamers won’t have the patience or energy to clear the game.