The reason I've heard is because Japanese phone companies had invented their own version of emojis on their own encoding scheme pre unicode, and to switch over, they needed unicode to also include support for these emoji, which is why a lot of them are very Japan specific.

Emojis then blew up with the rest of the world once people worked out how to enable them on the iphone. And since unicode has unlimited space for new emoji, there is little reason to deny any widely used symbol an emoji representation.