It's because Apple started including a new set of smiley things into their mobile device operating system back in 2011 and called them emojis because they were based on the Japanese icon set of the same name, then Android followed suit because they saw it was popular. Now most of the phones for both systems apparently have emoji icon things and refer to them as such.
So it's because phones are calling them emoji and most people are crazy attached to their phones, so they call all emoticon things emoji and it's catching on and spreading. It'll probably be the dominant term for emoticons by 2020, by my guesstimate.