Also in a recession a lot of trades slow down dramatically, because firms will be conservative. If not a lot of trades are happening, there isn't much to make money on. In addition to this, the strategies that the day traders relied on don't work as well, since the signals will change as people start trading differently.
I think another problem is that bear markets tend to be briefer and more erratic than bull markets, making your odds worse. The serial, consistent returns being long in bull market aren't as consistent as being short in a bear market. In bear markets it may just be best to sit it out.
