Several things.
One, they were large format cameras. Today we know film cameras as 35mm, but in those days, the photographer might have used a tripod-mounted box camera that made 4" square or even 4x6" negatives. You can put a lot of sharp detail in a negative that size.
Two, the lenses had a relatively small aperture for their focal length, which gave them an extremely wide depth of field- the area, in distance from the lens, that's in focus.
Three, because the film was so slow, you could, in some cases, actually walk through the scene without registering in the photo. A cameraman could then use a shielded lantern to selectively illuminate darker sections of the scene.
And fourth, as each photograph was individually printed and developed, a photographer could 'dodge and burn' the print to even further selectively over or underexpose parts of the image, which could bring up detail in "washed out" sections or dark sections.
Finally, unlike todays virtually cost-free digitals, a single photograph back in those days could cost the modern equivalent of $10 or $20 a frame. That's a pretty good incentive for the cameraman to get the shot right the first time.
Doc.