The word Celtic is basically associated with Ireland nowadays. However, Celts occupied a much bigger region in Continental Europe. For some reason, however, there's much more literature on Irish Celts and, to a lesser extent, British Celts than there is on, e.g., Celtiberians or Lusitanians. Non-academical sources are basically non-existent, and the only work of fiction about Continental Celts I remember is Asterix
If I had to guess, I'd say it's a mix of public interest and something the Roman Empire did, but I could very well be wrong
Thanks in advance
I think the key issue here is that the continental Celts seem to have ceased to exist in any recognisable sense by the fifth century (with the possible exception of the Bretons): by this point there is almost no evidence of continental Celtic being spoken; no polity arising from the Roman Empire had a Celtic identity; and no-one is known to have labelled themselves with a Celtic identity except as a geographic origin (e.g. the Auvergne in France remained a recognisable area, drawing its name from the Avernii of Caesar's time).
What we know of insular Celtic was basically recorded after 600, and often much later (the major sources of Welsh myth were written down in the twelfth and thirteenth centuries). Even Ogham, the Irish inscriptional script, is unlikely to predate the fifth century. So this information about insular Celtic society, language, history (albeit this is entirely post-Roman) and mythology all dates from a period when the continental Celts no longer seemed to exist as recognisable groups (their descendants clearly still exist).
It's worth noting that if we had to rely on Roman-era and earlier records for the insular Celts we'd have the same sort of knowledge as we do for the continental ones: a few scanty historical references, some more names in inscriptions, a fair number of place-names (supplanted by those identified by modern philology) and archaeology. It's also worth noting the same applies to the Germanic-speaking groups of western Europe before the fall of the Roman Empire: the lack of information on any group before they adopted a literate Christian culture that was not part of the Roman hegemony is standard.
It's also food for thought that we can only really understand the little we do about continental Celtic language because we can use insular Celtic as a guide. That is to say, not only does the survival of this insular Celtic allow us to know much more about insular Celts than continental ones, but it allows us to know much of the little we do about the continental Celts. So any study of Celts going beyond the archaeology of particular sites is likely to have to pass through insular Celtic language and culture at some point, and often requires the assumption that something recorded less than a millennium ago amongst insular Celts applied to other groups with similar languages over another millennium before...