Because what Twitter is, essentially, is a market researcher's goldmine. And access to real-time opinion. Why drop ads on people, when you can extract from them opinion that is probably more "valuable" than the small change from advertising?
Right now, they ask one question: "What are you doing?" Why not ask other questions? Rotate them, mix it up a little bit. They could even, for example, have a "word of the day" (or something) and ask people to tweet about that: in 140 characters or less, of course. Anything from a brand name to something like shoes. Re-package and sell that data to companies or competing companies. "This is what people are saying about ________ (you, your competition) today."
But that's just one idea; there's a lot of room to play with it. There's definitely a lot of untapped power/profit to be had in the collective.