Node.js is single-threaded because it uses an event-driven architecture, which means that it listens for events and triggers a callback function when an event occurs. This allows it to handle multiple requests concurrently, without the need for multiple threads.

In a traditional multi-threaded program, each new request requires the creation of a new thread, which can be resource-intensive and can lead to performance issues. By using a single thread and an event-driven architecture, Node.js can handle a large number of concurrent connections efficiently, without the need for multiple threads.