People were using emoticons, showing that emoji might be desirable. Some cell phone manufacturers/carriers added their own emoji [1]. They were very popular. Unicode added emoji to the standard, since lots of people were using it.
Not adding emoji would have been _more_ prescriptivist, and since it was desired anyway would have just caused fragmentation.
[1] https://blog.emojipedia.org/correcting-the-record-on-the-fir...