"I love my lapdock. It's easily the best piece of technology that I've purchased in the last ten years," said no one ever. And Motorola knows that, so they're getting rid of Webtop.
The reason (as if it's not already clear)? Lack of adoption, or in Motorola's words, "adoption wasn't strong enough." That's a nice way of saying "no one bought this crap." In all fairness, though, it makes sense, considering the direction Android is going in - ICS bridged the gap between smartphone and tablet, which basically eliminated the need for something like Webtop in the first place.
For those who may be new to the Android scene, Webtop was first introduced by Motorola with the Atrix 4G.