The log is sensitive precisely because it's thorough
An audit trail worth having records enough detail to actually reconstruct what happened — which necessarily means it also captures information about people and events that have nothing to do with whatever specific incident or request now requires the log to be shared outside the organization: an outside auditor, a regulator, an affected user disputing a specific action. Handing over the entire log resolves the immediate need at the cost of exposing everything else the log happens to contain alongside it.
Selective disclosure addresses this directly: sharing only the specific entries relevant to the matter at hand, with unrelated entries — which might reference other people, other incidents, or other matters entirely — redacted out before the log ever leaves the organization's control, rather than relying on the recipient's discretion not to look at or misuse the rest.
Redaction has to be built in, not applied under time pressure
Redacting a log properly, entry by entry, based on genuine relevance to a specific request, is considerably harder to do well under the time pressure of an active dispute or investigation than it is to design for in advance — a log format and access-control layer built from the start with selective disclosure in mind makes this a routine operation; a log never designed with this in mind makes it a rushed, error-prone one exactly when accuracy matters most.