There is no time to waste in short anime shows. The first episode needs to hook the watcher almost immediately, because there is no “it gets better in the second arc,” because there is only one arc.
Short anime are more focused, and the message is bright as day. With longer shows, the message can be skewed after a the first couple of arcs. The message changes as characters change, and that’s not a bad thing for long shows. Longer shows are more about growth and the journey the character (and the viewer) go through.
Short term shows on the other hand, have a set amount of time to tell one single story. As a result, you either get one really uninteresting tale, or one bombastically good one. Shorter shows have a single priority and they never lose sight of that.