A lead nucleus is much heavier than a neutron. Think of a billiard-ball colliding with a bowling ball -- the bowling ball is hardly affected and the billiard-ball keeps most of its energy even if it changes direction.

But a hydrogen nucleus is just a proton, and thus about the same mass as a neutron. So this results in lots of energy transfer, just as with billiard-balls hitting one another.

Like many plastics, polyethylene has lots of hydrogen atoms. Sticking them onto a carbon spine is a very efficient way to pack lots of hydrogen nuclei into a given volume of space.