In the West, we are obsessed with the arrow of time. Storytelling in the West is typically concerned with getting characters from point A to point B. The space of the story is of secondary importance to the one way flow of time. The here and now is of central importance.
This is not the case in Eastern cultures. Japanese culture has been heavily influenced by Vedic religions, as well Shintoism and Taoism.
A central concept of Vedic religions is Samsara[1], which can be thought of as a cyclical view of the universe. Time itself is merely an illusion, because ultimately the same cycle of birth, sustainence, and death is repeated infinitely.
That's why space is the focal point in manga. As another commenter stated, certain events are happening in the present. But the next scene may have non-linearly taken place in the future, or perhaps in the past, or even with no reference to the current time period. What ties everything together is the common space.