[pcottraux]

Further, why was Kong one of a kind ?

If you look carefully, when Kong is walking towards his cave/lair, you'll notice there is a giant gorilla boneyard surrounding him. Clearly Kong is the last of his species, although what happened to the rest of the Kongs is uncertain.

[ToddBush]

although what happened to the rest of the Kongs is uncertain.

Did you see how big they were? That's a LOT of gooooooood eatin'.