The source code repository for Explorer's league has less than a year of continuous activity. This likely is a relatively new project.
A short history is not necessarily a bad thing (all projects have to start somewhere!) but often, newer projects are changing rapidly and are thus less stable. They are also often innovative and exciting!
As this project matures, a longer source control history in conjunction with recent activity might indicate that the project has enough merit to hold contributors' interest for a long time. It might indicate a mature and relatively bug-free code base, and can be a sign of an organized, dedicated development team.
Note: The source code for Explorer's league might actually be older than the source control history can reveal. Many new projects begin by incorporating a large amount of source code from existing, older projects. You might be able to tell whether this is the case by looking for a rapid rise in the amount of code early in the project's history.
The source code for Explorer's league has not been changed in over a year.
Over 75% of all projects on Open Hub have no recent activity. Open source has a "long tail" of projects whose developers have moved on. But the code is still there for all to benefit from!