The main reason for this is because the sprite sheet that most of the Kirby sprites in the fangame are taken from was actually just a placeholder, and if the final game were developed, there probably would have been many more sprites expressing dynamic motion and direction.
From former DMA programmer James Watson: "The sprite sheet you see here were actually old placeholder graphics for the game, the proper graphics were cleaned up, larger (24x24) and the proper Kirby colour, unfortunately the SNES only dealt with 8x8 or 16x16 sprites and the sprite sheets for the improved sprites wouldn't have made a very good image since the tiles are all mixed up, the actual sprite was 24x24 normally, but could grow conciderably larger when you pulled Kirby back to full power, making the standard SNES sprite sheet even less viewable.
All the graphics for the game were rendered on SGi workstations (lucky artists, I had a horrible IBM Blue Lightning PC at work), and then hand retouched afterwards, this was quite a task given the number of directions you could pull kirby back in and the number of power variations you could use, it didn't show every direction/power combination but it was still a lot of frames just for kirby."