Biography depends on documents that happen to have survived. That dependence quietly determines which people can be written about at length and which cannot.
No papers means no book
A biographer needs letters, diaries, records, photographs or files to establish what a person did on particular days and what they thought while doing it.
Without them a writer can produce a portrait of a period with the subject in it, but not the detailed reconstruction the form is expected to deliver.
Publishers ask about sources at the proposal stage for this reason, since the archive determines whether the book is possible before anything is written.
Preservation follows institutional attention
Papers survive because someone kept them, and keeping them usually required a family with space, a university with a collection policy or an employer with retention rules.
People connected to institutions therefore leave records, while those outside them frequently leave almost nothing regardless of what they accomplished.
The imbalance compounds, because a collection attracts researchers, and their work attracts further donations to the same institution.
The gaps are patterned, not random
Working people, women whose correspondence was discarded, and communities whose institutions were destroyed or never funded are underrepresented in the surviving record.
Biographers addressing those lives work from oral history, census and court records, church registers, newspapers and employer files instead.
Those sources support a book but require far more time, which is a practical reason such biographies appear less often and later.
Access is negotiated, not automatic
Even where papers exist, an estate or institution may restrict who reads them, and permission can be conditioned on cooperation the biographer would rather avoid.
An authorized biography gains access and accepts constraints, while an unauthorized one keeps independence and works from what is publicly available.
Both approaches produce serious books, but the choice is made early and shapes everything the finished book can say.
Digitization is redrawing the map
Scanned newspapers, digitized registers and searchable public records let a biographer trace a life that previously would have required travel to a dozen buildings.
That has made lives outside the traditional archive far more researchable, and the recent expansion of biographical subjects follows the expansion of what can be searched.
The constraint has not disappeared, since digitization also follows institutional priorities, but the boundary of what can be written has moved considerably.