Vendors-have-no-capacity-to-handle-more-than-handful-number-of-emojis-per-each-release thing.

To elaborate: it should be plain obvious that not every Emoji proposal can be accepted even though all of them are correctly filed, as there would be too many Emojis there then. So there has to be some threshold, and that threshold is mostly stipulated by vendors' willingness to process new Emoji characters for designing fonts and updating softwares in time.