Most old video game consoles render games internally in 240p and then blank every other scan line. On CRTs, this effect works since the missing lines blend together with the filled in lines but modern TVs will often try to use video processing to 'fill in' these missing lines and even when they don't the effect doesn't look nice on a modern LCD or OLED.

Also, CRTs render each scan line in real time whereas modern TVs have to receive all 480 lines before they can render a frame. This introduces a non-trivial amount of latency into the video feed which can make certain old games unpleasant to play.