There are many reasons why Anime.js is such a popular library. To start with, it is quite simple to use. Everything is based on a single function call, and you only need to feed parameters into it, which is incredibly convenient.
Anime.js also has great documentation. You can see examples for each of the different animations available through this library that show you the code, how it works, and what it accomplishes through interactive animation visuals.
Finally, this library comes with an excellent learning curve. Anime.js is not challenging to learn at all! You can look at the documentation and come up with a nice animation of your choice if you have a basic understanding of CSS and JavaScript.