Why? Speed and flexibility of development (although tbh this is just about what you're used to), using the same language on both ends, and I really like js.
Not really, you could make that argument about any set of technologies working in tandem. Traditionally a framework is an entire set of technologies, only really designed to work together (e.g. Laravel, Rails, Backbone, CakePHP, etc.).
Express is a routing library for handling HTTP requests, it's not opinionated about anything else. It does one thing, and it does it fairly well.
React is a library for building UI components. It's not particularly opinionated on how you manage your data, or anything else really. It just renders things, and helps you do this efficiently.
Redux is a library for managing state changes in an application. It doesn't tell you how to render these changes, it just manages state.
These are not frameworks, they're focussed libraries.
Technologies? node.js + express; postgres; react + redux
Why? Speed and flexibility of development (although tbh this is just about what you're used to), using the same language on both ends, and I really like js.