I don't know much about them, but probably because history is written by the winners, and the Romans beat the Celts (??)
Why do so many contemporary Historians mistakenly believe that the Celts were somehow more "barbaric" than the Romans? Celts built roads, had somewhat democratic setups, and even advanced religions. So why do we today imagine them as savages?
"Barbaric", no way! I read that we see traces of the Celts every day. It was they, for example, who spread the use of trousers in the Western world; they were also the ones who invented barrels. There are other more visible evidences of their passage through history. In areas of Europe, you can still see hundreds of fortified hills, or hill forts, and burial mounds, or barrows, covering ancient tombs—all left by the Celts. Their enemies generally presented them as cruel, uncivilized barbarians. Today, thanks to progress made in Celtic studies, we can paint a very different picture of the Celts.