Emojis have gotten more and more inclusive but there is a quirk in the way Unicode handles emojis with some taking up more characters than others. This wouldn’t be much of a problem but with Twitter’s 280 characters, this might dissuade people from using the ones they identify with and go back to using the default colours and genders. You see in some cases, the more inclusive emojis end up taking more than two characters than the original yellow one. Emojipedia notes that an emoji on its own can take two characters but then you add a skin tone modifier that adds two more, and then you add a gender on top of that and that increases to nine characters all in all. What Twitter has done is treat all of these as two characters. It might be considered a small move but it does encourage people to use a more identifiable emoji in their tweets.
Source: The Verge