Java also has a very different development structure. It's owned and build by Oracle (RIP Sun), who get final say in everything added to the language. This additional red tape, and a smaller dev team working on both bug fixes and new features naturally lead to a slower release cycle.
Compare that to Node, which, while organized by the Node.js foundation, makes it very easy for people to contribute bug fixes and ideas. The core node repo is nearing 2,000 contributors. Having people help out, even if it's just with bug fixes, allow core developers to focus on new features. I don't think iteration speed is a sign of instability. Node has a well established release cycle and LTS process.