It's all digital these days, even in the mainstream churches. But I imagine that there's a better chance to guilt people into dropping a bit more into the contribution box if they're physically present. It's probably some psychological principle of which the WT is well aware.
Is trolley/cart ministry for the privileged few? I see a few people manning those in town. At the Sunday market two days ago I saw a rather substantial group gathered around the "Free Bible Course" sign. I haven't seen anyone door to door for a few months, though.