For people who don't understand making money:
If it sells out, it means that lots of people probably still want it and that they will very likely still buy it later. By having it sell out, Woot limits the amount of shirts that can be sold for 10 bucks when the shirt is really popular, so that they can sell more of the same shirt for 15 later. Chances are, enough people will still buy it so that you make more money even when the price is raised. Even if less shirts are sold at 15 than would have sold after "sold out" at 10, it will probably still be worth it in total revenue.
For shirts that aren't popular enough to sell out, having it a lower price for one day increases the amount of people who buy it by giving an incentive to those who may want it at some point to go ahead and buy it immediately. If I am not sure if I want a shirt, but I know I may regret not getting it, I may buy it because I'd rather pay 10 than 15.
That said, I really don't like this shirt and I'm surprised it sold out.