Because of course.
It’s an adage as old as ti- well, this generation. If a major game with online functionality release, it will have server issues at launch (unless, ironically enough, it is a Nintendo game). We saw it happen with Battlefield 4, DriveClub, Halo: The Master Chief Collection… and now, it is happening with Konami’s just released Metal Gear Solid 5: The Phantom Pain, which is being plagued with server issues even now, a couple of days since its initial launch.
Konami has been limiting online access and functionality to ensure things remain smooth, but for now, you might be better off simply playing the game offline- at least you won’t be dealing with the server issues then, and your game will run far faster.
Now just imagine if they had gone ahead with the original plan, and Metal Gear Online had also launched alongside the main game. It would have been a bloodbath.