It was because of a demonstrated demand for emojis with different skin tones to the default yellow. Before the Unicode Consortium had even considered this, there was a popular app called iDiversicons, which had similar emoji-like characters customisable with five different skin colour options, as images that could be copied by the user into their messages.
After trying and failing to convince Apple to allow their app to modify the iOS keyboard to make these easier to enter, the authors of the app took a broader approach, and wrote a proposal to the Consortium to include their emojis as standard:
https://www.unicode.org/L2/L2014/14154r-idiversicon-emoji-re... https://www.unicode.org/L2/L2014/14085-emoji.pdf
Based on this, the Consortium refined it into their own proposal that used the Fitzpatrick scale for skin tone modifiers:
https://www.unicode.org/L2/L2014/14172r-emoji-enhancements.p... https://www.unicode.org/L2/L2014/14173r-emoji-skin-tone.pdf https://www.unicode.org/L2/L2014/14174r-emoji-additions.pdf
And then, eventually, these were included in Unicode 8.0.