^^ Well, because it is stated in the manga.
From stephen's scripts.
[hide] Garp: Among the countless masses of pirates in the world,
he is one of the four greatest, along with the infamous Whitebeard, that dwell in the latter half of the Grand Line.
These pirates, who rule above all others almost as an emperor does,
are called the Four Emperors, or Yonko!!!
It is only Marine Headquarters and the Seven Armed Seas that can withstand their might!!
These "Three Great Powers" form a precarious balance that keeps the world from destruction, lest it fall. [/hide]