Gsquare beat me to it. In a nutshell, the difference between the speed of the solar wind compared to that of the comet is just too great for any ion tail arcing(sp?) to be perceptible. There's also the vantage point to take into account here. Pretty much always, we see the comet sideways from the Earth(usually, we see comets as they're traveling towards the sun (anyway, in my own relatively short(20 years) comet-gazing experience)). An arc, looking at it sideways, looks like a straight line. So even if a comet would have a perceptible arc, we wouldn't see it.