Mmm I'm not sure, I think the problem is that the pixels are not the same size, and the colors are too bright (obviously they should be a little brighter than the background, but not that much). The enemies also clash with the background, actually, and for the same reasons. It looks like you took the background from one place and the enemies and fruits from another. The bat does not look so out of place simply because it is darker, which hides the pixel size issue and is a similar brightness to the background..