The main reason is that it is low powered. An raster LCD - color or not - requires much more power and most of them need a permanently on back-light. New OLED display do not need the back-light but still pull more power than a segment-based LCD which has much fewer electronic circuits.

Most current top-plate LCDs mirror what is shown in the status line of the viewfinder, plus a few more settings which allow those to be set while looking at the camera from above. Each of these settings have a few states, so they do not require some that needs do be drawn from pixels.

It is only a matter of time, I would guess, because there are very useful things one can add. A color live-histogram would be great, or focus information, particularly for tracking. Now, anything that is possible is most likely to be implemented first on the rear color LCD. For this reason, I think that some manufacturers are likely to implement this on their high-end models to distinguish them as more advanced, almost just because they can.

Indeed Samsung did that on their Pro815 Ultra-Zoom and the top display not only showed quite a lot of information, it also serves as a waist-level viewfinder. Live-view on the top-plate, essentially.