One reason Rocket Goal works well as a casual game is that you can understand the basics quickly. You do not need to spend a long session learning complicated mechanics before having a decent match.
The short-match style also makes it easy to play during a break. You can play one match and leave, or continue for several rounds if you are having a good time. There is enough room for small improvements too, especially when you begin to understand how movement and shooting work together.
Another enjoyable part of Rocket Goal is the unpredictability of each match. A game that looks comfortable can change after one missed shot or a quick counterattack. That keeps you paying attention instead of simply repeating the same actions.