The real explanation is more fundamental. It has nothing to do with psychology.
There is a fundamental asymmetry between long and short positions. Because of limited liability and all that corporate personhood junk, there is a terminal point on the short side of a stock: zero. Once a stock hits that, there's no coming back. An upward movement is always subject to a possible correction or reversal. A down movement isn't.
Thus, when the bull is out scouting for oversolds he has to consider the possibility (especially in the case of financials that make heavy use of short term debt) that the overly bearish sentiment will turn out to be a self-fulfilling prophesy. It doesn't matter if the bull has perfect information; he still has to worry about the droves of underinformed bears.
Another asymmetry is the fact that bears have always had fewer options than bulls. In particular, it's usually impossible to short a specific instrument over the long term. So while the bears inherently have more power than the bulls due to business law, because of market laws they can exercise it only via derivatives: short term, highly leveraged bursts vs. the tens of millions of 1:1 leverage, buy-and-hold (and-occasionally-panic) bulls.
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