1) The UI+UX is superior, 2) it evolves very quickly, often as a response to user requests, 3) it is entirely free/follows a libre philosophy. The importance of point 3) varies between individuals. Where the competition shines is in the presence of top/titled players, but even there lichess is progressing quickly.

Well it's open source (https://github.com/ornicar/lila), and all features are for free, forever, without any ads. That certainly puts it apart form other chess services I know.