Well, two reasons:
1. The company is run by people who are used to making money by hiding behind TV monopolies created under the frederal government's Canadian content policies. They don't actually know how to run a real media company. They just pretend to. A great model in 1990. Not so good now.
2. As the Globe's Andrew Willis suggests, the stock is probably overvalued. Those people who sold it short (like shooting fish in a barrel, they bet against Canwest stock in the TSX crap game) still have to buy stock to complete their contracts. Canwest stock's real value (assets minus corporate debt divided by shareholders) is probably zero. The chances of the stock surviving a "restructuring" and being reborn as real equity is slim to none.
That doesn't mean the TV stations and the newspapers are done. Far from it. They are, in fact, cash cows. But when the dairy farmer has recklessly borrowed against Bessie, then Bessie gets her bovine ass sold. Right now, there are some guys from the next concession giving Bessie the once-over.