Because it's a Japanese-specific phenomenon (isn't it ?), and I would expect that different language Wikipedias copy from each other all the time, so if your goal was to (as I assumed it to) to put that information on (at least) the English Wikipedia, then having it first featured on the Japanese one would give it much more weight ?
Emoji is of Japanese origin but absolutely a worldwide phenomenon at this point!?! Of course this new info should be on all wikis, but I don't think it matters at all which one comes first. I don't edit Wikipedia, so I don't have any skin in the game.