Edge LED lighting in LCD TVs like yours allows for a thinner bezel. The problem though is achieving uniform lighting across the whole panel. The thinner the bezels are, the more challenging it is. In short, you cannot achieve uniformity with edge lighting, and therefore your contrast ratio and overall PQ is lesser.
To test this, create a 1920x1080 black image in MS Paint with a small/medium sized white cross/circle in the middle. Play this image on your TV in the dark with the lights off. You will see what I mean by non uniform lighting, white spots, light bleed, etc.
Compared to plasma, where each pixel is self illuminated. This means each pixel can turn off or on. It does not use a backlight or LCD matrix like in LCD TVs. One of the many reasons why plasma has a better contrast ratio and better PQ.