Originally Mastodon didn't have an official app, everyone used third party apps like Tusky and Toot!
The official app was only introduced last year, and it was missing a lot of important features because it was intended as a sort of "Fisher Price My First Mastodon" kind of experience.
The third party apps have a lot more care and attention paid to them, and some have been developed for longer.
Also, due to the open standards on here, official apps have no special privileges.