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Comets move rapidly (relatively) across a given field of view, (discounting the regular rotation of the earth, of course.) while (relatively) galaxies and stars are stationary (though, over long periods of time, they will move in apparent location across the sky)

And galaxies, while much further away than a given comet, are, of course, intrinsically billions of times brighter than a comet.

And astronomers really don't spend that much time looking for comets; the research money and prestige is in studying stars and other galaxies.

Other than a couple of satellites that look for comets (such as NEAT) most comet-searching is done by AMATEUR astronomers with small telescopes in their backyards. Hubble, Palomar, the Keck telescopes in Hawaii aren't wasted "looking" for comets.

So, it's not surprising at all for any comet to slip into the inner solar system before being discovered.