Historically, documentation was the responsibility of the Engineering department. The task often fell to junior staff members or others who could be spared to comply with the company's contractual requirements.
The customer support that documentation provided, was written on an as-required basis by harried personnel. Engineers, developers or other technicians who undertook documentation often did so begrudgingly, seeing it as a distraction from their main rôles in development and production.
Understandably, authors write for their peers. In this situation, however, the peers were both technically conversant and usually already familiar with the product.
This process often produced a document that frustrated the user and bogged the Engineering department with customer calls for support.