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.

Generally Unicode is for encoding all existing encodings/writing.

So you generally can’t add something because it would be cool or fun or useful, but only because it is currently in use and cannot be encoded by Unicode.

That list only includes suggestions that were seriously considered and voted on.

Since it's a vote, there is no single official 'reason' for rejection. If I had to guess: it would be confusing to anyone who didn't grow up with American TV shows.