https://news.ufl.edu/archive/2012/02/color-is-key-in-controlling-flies-uf-researchers-find.html
A key to making the device effective was the discovery that flies are three times more attracted to the color blue than to yellow and that yellow actually seemed to repel flies.
Well TIL, maybe it’s the color?
My original guess was that red watermelons are more common and so flies have evolved to crave it. You could also try with other fruits and report back 😄
Here it is
https://lemmy.ca/comment/11489606