What accounts for the Kuiper Cliff?

The Kuiper Belt is believed to be what is left of the outskirts of the protoplanetary disc from which the Solar System was born. The small number of objects still present in the inner belt shows that most of the objects were ejected, possibly due to the outward migration of Neptune. This should have left the outer belt intact, but the number of objects beyond 45 AU is virtually zero – called the Kuiper cliff.