This may be American-centric since they might be as known in European countries, but it just feels like we hold the gods of ancient Rome and Greece of more value than the rest of Europe.
The Roman Empire probably had a role in that but with how powerful the British, French, Dutch, and Spanish would become, it doesn't feel like they embraced their ancient beliefs as much as Italians did like during the Renaissance period.
I know many Christian traditions like Easter and Christmas was adopted by pagan traditions but again, it feels like Americans don't hold the same value of importance of the European Celts than the Romans and Greeks. Their gods just feel like they have become household names to people unfamiliar with their culture would still know from pop culture entertainment that I don't see from other European cultures.
The Greeks and Romans have a large pre-conversion body of literature, written by people who knew the stories and the pre-Christian religion firsthand. Pre-conversion Celtic religion is known largely from Classical observers, meaning that it is scant, secondhand, and requiring of source criticism. Post-conversion Celtic sources provide some insight, but these, too, require considerable source criticism.
Also, because Europe looked to the Classical period as "classic" for lack of a better word, there was considerable interest in all things Greek and Roman. The Celts were not look upon during most post-Roman periods as having the same stature as Classic civilization, so there was considerable interest in keeping he Roman and Greek gods as part of the common cultural inheritance.
I'd also add that Northern traditions seem to have suffered under the undeserved defamation that the Middle Ages in general suffered, the consecuences of which we still see today in most people's conception of the period and the way it is depicted in popular media.