Unfortunately, the founder of ZeroMQ, Pieter Hintjens passed away (due to cancer) [1]. He was a regular on HN [2].
ZeroMQ still works great and the open source community is still maintaining it on GitHub [3]. I just think people are also looking at other technologies. A lot of interest popped up in things like Apache Kafka and Samsa. I still think ZeroMQ holds a unique place due to its lightweight and simple nature.
I have been curious how the community would hold up after Pieter's death. This project is a unique case because of how much work went into building community and welcoming contributions. That said, the world is a different place than in zeromq's heyday. Other commenters refer to Martin leaving the project, C++ regret, and a poor fit with node.js. Maybe in the face of all those changes zeromq's mature community is primarily why it lives as a project.