Because it only makes sense that after 10k years at least 1 marine chapter realized that giving their giant walking slabs of metal another tiny piece of metal triples its durability.
How this works noone knows. The Mechanicus are also still trying to figure out why the shield only blocks from the front when the shield arm could never physically be able to aim the shield anywhere but left.
The real reason is that Storm Shields are in fact Draigo's toe nail clippings and retain a portion of his plot armour. And the Shield Eternal is simply an autographed toe nail clipping.