There are plenty of strategies that can be profitable on a small scale but which just don't work as you scale up the capital or leverage. Such strategies can be simultaneously profitable for a small operation and not worth the bother for most larger trading shops.
This is just one example, but at a small scale you might be placing buy orders with let’s say $1k or $10k. This is insignificant compared to the total amount of money being traded, and will not affect the stock price.
However if you start increasing scale to $1mm or $10mm, your buy or sell orders begin to actually move the stock price itself. You might not be able to successfully sell $10mm of stock without dropping the price, signaling others to sell, further dropping the price, cutting into your own profits.
For the point I'm trying to make, the particulars of the strategy don't matter. What matters is the fact that the larger and/or more numerous your orders are, the more likely you are to move the price, and any such price movements will necessarily be disadvantageous to you.
I.e., if you're buying, the larger your buy order is (well, assuming a visible buy order) the more likely it is that liquidity-adding sellers will increase the price of their sell orders. Also makes it less likely that liquidity-taking sellers will want to trade against your large buy order, because (like everyone else) they'll tend to interpret your large buy order as a sign that the price is likely to increase, so not as good a time to sell.
You could of course use hidden orders to avoid some of those disadvantages, but hidden orders have their own set of tradeoffs too.
