There is a lot of anime series that were only shown for one season because they were not able to get the attention of a lot of people. There are also some that have a successful first season but the second season has not been released yet. This particular anime series may be one of the first ones that people will watch.
This is easy to love so it is disappointing that the second season has not been released yet. This is a bit of a surprise because of its popularity but it has changed animation studios recently. They are making sure first that everything is properly accounted for before releasing the second season.
One Punch Man is a newer anime with only one season out currently. Although some anime can take several seasons before fans become aware of it, One Punch Man is wildly popular and left fans demanding more. Many people were confused about why One Punch Man season 2 was delayed since it has become so popular.
The answer though is totally understandable. Season two was only delayed because the series was passed to J.C. Staff from Madhouse which are animation studios. Since it changed ownership, they delayed the second season but promised that there will be a released 2nd season soon!