Short answer: a lot of added complexity for no noticeable net benefit in traction or performance - and in fact lower performance than a comparable MTB due to the added weight and drag.

I suppose someone could find a trail on which 2WD might possibly be of benefit, such as a 20+% grade with loose rock and scree (a trail of this type exists less than 2 miles from my house), and in such places it's easy for a standard 2" MTB rear wheel alone to lose traction. When I tested the Legacy, I lived elsewhere, and was not aware of this trail. But this presumes a rider could apply weight/vertical force to the front wheel in a manner that would prevent it from slipping and skidding (which would be a Bad Thing).

A different solution (wider rear tires / "fatbikes") has appeared on the market, which has seen much wider adoption as it offers other benefits in addition to traction.