Because it has better moves.
Normal type moves are really only used when they get STAB becuase they don’t beat anything. Leafeon has Swords Dance, X-Scissor, Leech Seed, Leaf Blade, Wish, Curse, Synthesis and other moves that are just better than body slam. The extra effect occurs less than a third of the time, so it’s not the most reliable move ever. If you want speed, use it on a sun team with chlorophyll