We use React because we can build complex user interfaces through composition of stateful components. It allows us to build UIs faster, through component reuse and there is a wide range of community maintained components available to build upon. React components are easily testable which gives us an assurance of quality.
We use RabbitMQ because we need to utilize messaging at various different places in our infrastructure. From real time message ingestion, to asynchronously reacting to user actions. It uses AMQP, it's easy to set up and manage and having it installed and set up on our instances prevents vendor lock in issues.