In order for Wikileaks to continue to operate over the Internet without being hijacked by the people that control whichever segments of the Internet that Wikileaks is currently connected to, they have a digital secret in the form of a public-private encryption keypair.

Using the private key to produce a "signature" value of a digital item demonstrates that the person who holds the public-private keypair was in possession of the digital item at some point, and that the exact copy of that same digital item is what you currently have in your possession.

Recently, Mr. Assange's access to the Internet, and possibly his person and his computer (which would contain the secret private PGP key used for signing) were very possibly compromised by state actors.

It may be possible that Mr. Assange has been / is being coerced to hand over all secrets that are encrypted and sent to him.

It is understood that producing signed messages is only done if the signer is reasonably sure that their person, systems, and secrets (including the private key) are not compromised.

If Mr. Assange and his computer and private key are compromised, and he is being coerced by any third party, then the only viable recourse he may have to resist them is to "forget" the passphrase for his key, and for the fallback keys that may exist.

If Mr. Assange is unable to produce a signed message, using a key in Wikileaks' established trust fallback lineage, then we must assume that his person and systems are compromised by a third party and that therefore the mission of Wikileaks is compromised.