Compounding works in your favor when you are long, and it works against you when you are short. A losing long stock position eventually becomes a negligible part of a portfolio while the winners become gargantuan.
You also have a lot more flexibility with your entry strategy when you are buying than you do when you are shorting. You can't effectively martingale a short position, but you can with a long position. On the flip side, you can't pyramid into a short position anywhere near as easily as you can pyramid into a long position: your short position gets smaller and smaller as you are correct about it, meaning you have to sell like crazy just to keep a good size, whereas a winning long position will get larger and larger organically, so you can add to it modestly for huge effect.