Biographies commonly take a decade or more from commission to publication, far longer than most other non-fiction. The delay is caused by the nature of the evidence rather than by the writing.
The sources are scattered by default
A life leaves traces across employers, institutions, correspondents, local records and the private holdings of relatives, none of which were assembled with a biographer in mind.
Locating those holdings is itself research, and access frequently requires permission from an owner who has no obligation to grant it.
Archives must then be visited in person, often abroad, and material is released at whatever pace the holding institution works.
Uncatalogued collections take longer still, because the biographer has to read through material nobody has sorted in order to discover what is there.
Interviews depend on other people's willingness
Witnesses to a life have to be identified, approached and persuaded, and many will decline or agree only after long negotiation.
Those conversations frequently produce contradictory accounts of the same events, which sends the biographer back to documents to establish what can actually be supported.
The people who knew a subject best are also the ones most likely to be elderly, which places a limit on how long the work can be delayed.
Verification is slower than collection
A biography's authority rests on its footnotes, and each claim has to be traced to a source that can withstand challenge.
Recollection is unreliable, published accounts repeat one another's errors, and a single unsupported assertion can undermine a reader's trust in everything around it.
Checking what a subject actually did on a given day can take longer than writing the chapter that mentions it.
The structure cannot be planned in advance
A biographer does not know what the book argues until the research is substantially complete, since the interpretation emerges from the material.
Drafting therefore begins late, and discoveries made during writing routinely send the author back to sources they had considered finished.
Deciding what to leave out is equally slow, since a life researched thoroughly produces far more material than any publishable length can hold.
Publishers plan around the reality
Contracts for major biographies allow long delivery periods and stage payments accordingly, because everyone involved expects the schedule to move.
The alternative is a quick book assembled from published sources, which can be produced in months and is a different product entirely, whatever the shelf it shares.