Because you need enough room under your microscope to accommodate the lengths of your instruments and you want want to consider how much anatomy you want in your Field of View. For ophthalmic procedures, fixed focal length convergent lenses are used. Usually f=175 or f=200. Some operating microscopes for ENT procedures also may use a fixed focal length front lens.
For neurosurgical, spine and ENT procedures, the focal length is variable. For example, the objective lens combination can be configured so the focal length is from f=200 to f=450. So any object within this focal length will be in focus. Any object outside the focal length will never be in focus. The working distance from the surgical site (the objective) can range from 200 to 450 mm. So if you use an instrument that is 300 mm long, you may not want the microscope focused at 200 mm.
Also, remember that with increased focal length (working distance) more of the objective will be in view. This is called Field of View (magnification aspect discussed later).