Because a Little Bug Went Ka-Choo! provides the same thrill as watching a Rube Goldberg machine in action--or seeing a million dominoes falling consecutively from the push of a tiny finger.
Starting with the sneeze of a little bug, a chain of events takes place that ends in a chaotic parade. The realization that each action is causing something progressively larger builds excitement for the satisfying conclusion (which shows, though doesn't state, that small actions can have big consequences).
I wonder: How many millions of kids will get to hear this exhilerating story because a loving parent with a soft voice read it to their little kid a long, long time ago?