Because it didn't look good at the day, even if it was state of the art.
But even by modern standards, CRTs are widely considered better looking for games. Not just retro consoles where the sprites were designed to take advantage of the blurring effect (thus using the limitations of the display to create new colours and emulate alpha blending) but also many modern gamers are starting to see the appeal of CRTs too because it means they can run their games at lower resolutions without them looking pixelated and turn all the rendering effects up to max.