Ever since Metal Gear Solid 3, the franchise has become increasingly focused on the exploits of Big Boss--the man responsible for the creation of Solid Snake. Both Metal Gear Solid: Ground Zeroes and Metal Gear Solid 5: The Phantom Pain will focus on the older Snake.
One of the reasons why creator Hideo Kojima returns to Big Boss is that he finds it "easier" to tell his story. "As you guys know, Solid Snake is not a real human, but Big Boss is," he explained to us.
"So we really see emotions from him," Kojima pointed out. "Story-wise, it's easier to create stories with true emotions for that character."
The upcoming series of MGS5 games will take place across 1975-1984, having Big Boss inch ever closer to his eventual confrontation with Solid Snake in the original Metal Gear. He'll be known as "Punished Snake" in The Phantom Pain.