A chart of the market since that time shows a very steady climb upward. Shorting goes against that grain, against what has been the natural movement of the market, and also tries to do so in the shorter-term. Market timing can't be accomplished, though, of course. And while you can argue that a person isn't shorting the market, but is only shorting a certain stock, even company-specific timing is very difficult to do. When you short, management is working against you, progress is working against you, and business is working against you.
Business is a creature that looks for ways to grow.
When you analyze stocks to buy, you know what management is aiming to do and you see what they have done thus far through their past performance. That doesn't mean that they won't execute in the future, though. That's what they're working to achieve. And you don't know how well they can execute once they begin to, because you haven't seen them do it in the past. Shorts claimed that the business model couldn't be profitable.