The glowing Apple logo was caused by the backlight shining on the back of the screen, as well as a hole cut in the back of the lid (in the shape of the Apple logo) with a plastic Apple-shaped cover through which the light also glowed. If you turned off your screen and let the light shine through the back (like sunlight), you’d see the light shine through the Apple logo as well.
When they switched from having a glowing Apple logo to not having one, they also switched over the display technology. There was no longer any backlight to illuminate the screen as there had been previously. It used to be that a fluorescent backlight tube would shine at the bottom of the screen and light up the space behind it. It is now with a mask layer and for structural reasons as well, as the lids are significantly thinner.