I guess the current level of contrast is the best they can manage out of e-ink panel and controller. I think this is a reasonable assumption since major contrast improvement has been achieved with new e-ink panel (Vizplex over older ones used in PRS-500) but devices using the same panel do not vary that much in terms of contrast.

If it is the case, offering more options will mean offering options to reduce contrast. Given that many people are not quite satisfied with the best possible contrast, I don't see much point of offering options to reduce the contrast.