[M_a_t_t]

Do you mean why is no one putting new transmissions in older cars? or why don't car manufacturers do it?

It sounds good initally, but the cost for the average ecomodder, let alone "average" person, wouldn't make sense for gas savings. The transmission would probably be 300+, then an aftermarket controller to control it would probably double the cost. Not to mention any changes needed outside the transmission swap like driveshaft, mount, etc.