Chatbots proposed to solve a problem that current technology is bad at: understanding natural language. Very few experts claimed that algorithms could master the combinatorics of natural language beyond a few very narrow domains. On top of the technical risk, the voice and chat UX does not give a lot of visibility to users, as this post points out. What happened was: algorithmic advances in other areas lead to analogous reasoning by non-experts whose self-promotion aligned with the media's need for something new, big and understandable. And then it failed.

The irony, of course, is that research is making strides in NLU, it's just too late for the last wave of chatbots. Here are two recent papers from DeepMind:

Learning to Follow Language Instructions with Adversarial Reward Induction https://arxiv.org/abs/1806.01946

Relational inductive biases, deep learning, and graph networks https://arxiv.org/abs/1806.01261