Emoji was supported for a long time on Japanese phones. Apple wanted to sell iPhones in Japan, so they implemented emojis using Unicode's Private Use Area which initially was a feature only available in Japan, but people quickly figured out how to use them outside of Japan.

Emojis became a rather popular feature of iPhones, and Android wanted to have emojis as well, however using Private Use Area just like iPhones did wasn't ideal, so they made a proposal to Unicode Consortium to encode emojis in Unicode.

Later, there was a lot of demand to add new emojis, so more and more emojis got encoded into the standard.