Lead makes things worse, not better. High-energy particles go straight through a couple mm of lead no problem, and lead itself is radioactive anyways. The problem is when a particle punches straight through a chip, leaving some energetic charge behind. You won't stop that with a paper-thin layer of lead.

Also, lead is extremely dense.

Because lead turns a single high-energy particle (that would disturb a single bit and punch through) into a shower of many low-energy particles (that would disturb many bits AND induce lattice damage over a wide area).

lead being heavy, I wonder if that tradeoff wasn’t worth it?