I think it's the gritty look of it.
The "simplicity" is appealing, but perhaps the sense of gloom might be a touch too much, diverting love away from respect? But then isn't that because of the era? Maybe doing that stuff when all around were shooting 8x10 on angel skins might have been part of a strategy to create a difference of identity.
Again, so much was done in darkrooms that nothing is sure. Just consider Lillian Bassman!
However, whatever it was, it worked for him!
Rob