Those old valve colour TVs used huge amounts of power and generated lots of heat. The transformers particularly the line scan and frame scan transformers were huge things and some of the voltages generated were massive. They were not really designed to be on for hours on end, but at the time of course TV transmissions were just starting to allow just that. With the advent and development of transistors and solid state circuitry TVs used much less power, generated hardly any heat and became lighter and more compact. The next technological leap saw the cathode ray tube, and the required massive use of power to scan the images across and up and down the screen with scan coils, replaced by the plasma and now led flat screens. So with lower voltage and current use plus better insulating materials modern Color TVs are much less prone top go incendiary.