I'd also guess because (potential) mechanisms to do so would be attacked on forums like HN because they sound too close to crypto/web3 and most people wouldn't bother to separate ideology from technology and create a PR nightmare for internet archive. So it's probably harder to recruit people to work on it.
Its also a really hard issue. My fav is IPFS, Its the most common suggestion but it only works on paper. IPFS Allows people to pin things, but it still has to be stored /somewhere/ and, it gets really damn slow once you throw more then a million files at it (I know... we tried)
Overall, Its just a giant archive, its hard to store that much data in a cheap manner that will last 20 years.
Just do something similar to folding coin, which is used for things like protein folding. Perhaps an Internet archive coin that can be earned by hoarding backups of the archive on hard drives.
See Filecoin Foundation and their work with IA.