Incidence angles of meteoroids
Horizontal atmospheric entries are unlikely because the object would have to almost miss the Earth and brush against it rather than impact it (and probably would disintegrate in the atmosphere if it's a small one, due to the long flight through it in such case).
Vertical entries are unlikely for the opposite reason: the object would have to fly at precisely 90 degrees relative to the Earth, meaning the Earth is in the very center of the object's orbital path.
But if an asteroid or comet comes at another angle its path may be changed by the Earth's gravity towards about 45 degrees. This is an approximation, it is about 40-50 degrees, but 45 degrees is the most likely one out of specific angles.