I used to love Mocha + Chai combo, but as time flew, I felt it's limiting. Other projects like Jest shown that there is room for innovation in this space. With last version published 2 years ago, Chai seems abandoned. Furthermore, as TypeScript becomes more and more popular, it became evident that some things about writing assertions could be improved. earl is an effort to bring a little bit of innovation in the space of assertion libraries.
Why not just Jest?
I really enjoy some of the Jest's features — that's what inspired this library in the first place. However, I really hate others. Jest feels too magical and full of bugs for my taste. On the other hand, I always enjoyed simplicity and confidence that Mocha provides.
Simply put, Jest takes control away from you, Mocha puts you in charge.