Hostel booking sites order results by what earns them the most. This links to the nonprofit federations that invented the model and still run much of it.
Hostelling began as a nonprofit movement and large parts of it still are. That matters practically: a Hostelling International property answers to a national association with standards, while a listing on a booking platform answers to nobody except the platform. Neither is automatically better, but knowing which one you are looking at changes how you read the reviews.
The reading list
Link-out only. No booking sites and no affiliate links.
Where hostels came from
A German schoolteacher, 1912, and a nonprofit federation that still exists.
The federation's history and structure: a network of national associations rather than a company, which is why standards vary by country and why membership works the way it does.
The federation's own booking and directory site. It is selling beds, but as a nonprofit membership body with published standards rather than a commission-ranked marketplace.