If you were wondering why Kojima Productions is splitting up Metal Gear Solid 5 into two games, it has everything to do with how long we'll be waiting to play Phantom Pain.
In an interview with Famitsu (translated by Siliconera), Hideo Kojima discussed the decision to split MGS 5: Ground Zeroes and MGS 5: The Phantom Pain into two different titles. “Prior to the announcement of Metal Gear Solid V5, last year, we showed a Fox Engine implementation video,” Kojima said. “Then, we also showed the Fox Engine in a presentation video at GDC 2013. Since we revealed information on a regular basis, there were many who believed that Metal Gear Solid 5 was already complete, and there were many voices from fans saying ‘we want to play it soon’.”
While that's not really a possibility due to how long it's going to take to get Phantom Pain finished, Kojima said the decision to release Ground Zeroes came from the fan outcry. “However, The Phantom Pain has enormous volume, so it will require much more time,” Kojima added. “So, together with the marketing staff, we consulted ‘what ifs’ and such, and came to the decision of allowing fans to at least play the prologue, ahead of time."
As for when we can expect to finally get the full version of Metal Gear Solid 5, Kojima didn't reveal an exact date. However, he did hint at a possible release window, which just happens to be incredibly vague. “It would’ve actually been better to release it all at once, but by the way things are going, it appears likely that Phantom Pain will be coming out around the time the PlayStation 4 becomes more spread out," Kojima said. There's no telling just how "spread out" the PS4 has to be for Phantom Pain to see release, but the console is off to a good start with 2.1 million units already sold.
At least we'll have Ground Zeroes to look forward to early next year.