Comets: How Big A Threat To Earth?

Earth-bound asteroids grab newspaper headlines for good reason. Scientists say the fallout of an asteroid several city blocks wide smacking into the planet would be catastrophic. Mass extinctions, runaway infernos, erratic climate fluctuations, and devastating impacts on human civilization are just some of the scenarios imagined.

Why, then, does the threat of a comet impact with Earth–potentially as dire if not worse than an asteroid–rarely leak onto the pages of the popular press?