Physics prohibits the existence of one-way mirrors. If you make a transflective screen, it will also reflect less light than a purely reflective screen.

So it is better to make either a purely emissive (OLED) / transmissive (LCD) screen, or a purely reflective screen with an added front light.

And why are there so few devices with reflective LCD screens? They are quicker, but they reflect less light than the electrophoretic screens from E Ink.