Cart work is the JW equivalent of an elitist sport, you have to qualify to be worthy enough to do it. The idea of cart work as a replacement for d2d doesn't fly unless everyone in the cong has a cart and that just ain't gonna happen ever.
My theory about the carts is that the GB introduced them as a new carrot to dangle, an outward tangible show of "change" to boost moral and to convince the mindless masses that the chariot is indeed moving forward innovating new preaching methods etc etc blah blah.
But, in reality the cart work is just meant to be a distraction from the fact that they are changing the way they exist as a religion in such a major way that they now will freak people out and they do not want to spook the people too much before the new WT is complete and will is gradually revealed.