Not a simple explanation, I don't think. The size of the smallest spaceship with a given speed and direction is an emergent consequence of iterative rules, so

Not a simple explanation, I don't think. The size of the smallest spaceship with a given speed and direction is an emergent consequence of iterative rules, so explaining "why" for a Life spaceship is like trying to explain the Seahorse Valley in the Mandelbrot set. In both cases, the iteration rule is simple, but the consequences are complex -- no pun intended.