It's really hard for shorts to execute because of the way that the price feed lags behind external market prices.
In an up market:
- Price of bitUSD increases in terms of BTSX
- Feed lags behind so people cancel their short positions
- Only shorts that execute are those who exited their positions late, and they end up eating an immediate loss.
In a down market:
- Price of bitUSD decreases in terms of BTSX
- Feed lags so buyers cancel buy orders, or sellers can go under the feed price
- Almost no shorts end up executing
I think this is one of the reasons that bitUSD cap is so low right now, because short sales are necessary to increase the number of assets.