Partly the reason is contrast. The light reflecting off the surface is much brighter than the tail itself, if you wanted to catch the tail it would overexpose t

Partly the reason is contrast. The light reflecting off the surface is much brighter than the tail itself, if you wanted to catch the tail it would overexpose the nucleus. Besides that, the comet tail is formed from the outgassing from the Sun-facing surface of the comet and then wrapped up in the magnetic field of the solar wind, so the bright stuff you see in the sky is usually something like 10 thousand (or more) kilometers away from the actual nucleus, the bright part you usually see from Earth around comet is the coma (nebulous gas) not the solid surface of the comet, with the tail extending out many hundreds of thousands of kilometers behind it, though technically the tail will extend as far as the solar wind carries it once it's entrained in it :)