The primary source of uncorrelated bit-flips is, effectively, cosmic radiation.

(Correlated bit-flips are likely from a manufacturing error... or, at least in one case, from excessively radioactive ceramic packaging.)

The more atmosphere you have to randomly absorb high-energy photons, the better.

Higher density memory drops fewer electrons in each well, which means a lower-energy photon can change the state.

Corolary: submarine datacenters are much better than orbital datacenters. Cheaper to cool, construct, shield and access.