I'm guessing that aerodynamics more than anything else. I'm not sure how much drag there is from a traditional exhaust. But I'm thinking that the undertail does not eliminate nearly enough drag to justify itself and its high cost. People probably just thiunk itlooks cool. That, and stupid passengers will stop burning their legs.
The biggest plus for the manufacturers is that they capture a whole new product market by putting an undertail on - this separates the bike with the undertail from the previous bikes with the traditional exhaust. It magnifies the feeling that the bike is part of a "new generation" and thus more appealing to squids the world over.