I've been around practicing Jews, including gefilte fish eaters, all my life, and have never heard of such a tradition. Many Jews eat gefilte fish weekly or even more often; many never eat it at all. I've never heard of a practice of eating it specifically on Passover. Indeed, many Jews won't eat any fish — gefilte or other — on Passover.
I can think of three possible answers to your question, then.
- You're wrong. There's no such tradition. You may have been influenced in your thinking by the fact the supermarkets carry a greater variety and quantity of Jewish-style foods on and before Passover, including gefilte fish, and therefore you're associating it with Passover.
- The tradition is a new one, caused by the supply-side consideration in the previous bullet point and/or by people's generally feeling more traditional (wanting to return to their gustatory roots) at Passover.
- The reason people don't eat fish on Passover is that, in Europe, fish were kept alive by putting whiskey-soaked bread in their mouths (see e.g. Taame Haminhagim, footnote 7 to paragraph 478, who discusses this issue though he doesn't mention not eating fish at all). This can sometimes render the fish non-kosher for Passover (as bread and whiskey are). There is more room for leniency under some circumstances (as also described there) if the fish was bought before Passover began. If you're right that there's a tradition of eating gefilte fish on Passover, then I conjecture that it may stem from having older fish on Passover, bought before the week-long holiday began, which would last longer fresh if turned into gefilte fish than if eaten whole (or people may have thought so, or at least the added ingredients would mask the bad flavor of fish that's going bad). Just a conjecture.