I would think the main fiscal reason it has not been mass produced is the instability of the world price of lead. In the last two years it has gone through some amazing changes. From $0.40/lb. in 2005 to $1.82/lb at the end of 2007. Make no mistake, to get the power density and reliability required for this, you need nearly pure lead. That is not cheap right now.

NiMH requires special charging care and Lithium in most applications is dangerous. The configurations of Lithium that are not dangerous are very $$$$$.

The supercapacitor idea sounds cool though.