Why Node.js® - several reasons of nodejs popularity?

Node.js®, known as Node is gaining attention of designer. Node has been proved as good option to write highly scalable network solutions. We recently choose Nodejs as our server language.

Asynchronous event driven JavaScript runtime 

Node is designed to build scalable network applications. Using nodejs many concurrent connection can be handle. On each connection the callback is fired, but if there is no work to be done, Node will sleep. This is opposite to common concurrency model where OS threads are employed. Thread-based networking is relatively inefficient and very difficult to use.

Node are free from worries of dead-locking the process

Users of Node are free from worries of dead-locking the process, since there are no locks. Almost no function in Node directly performs I/O, so the process never blocks.
Because nothing blocks, scalable systems are very reasonable to develop in Node.

Similarity with Ruby's Event machine and Python's twisted 

Node is similar in design and influenced by system like Ruby's Event machine and Python's twisted. Node took the event model a bit further. It presented an event loop as a runtime construct instead of as a library. In other systems there is always a blocking call to start the event-loop. This behavior is defined through callbacks at the beginning of a script and at the end starts a server through a blocking call like EventMachine::run().

In Node there is no such start-the-event-loop call. Node simply enters the event loop after executing the input script. Node exits the event loop when there are no more callbacks to perform. This behavior is like browser JavaScript — the event loop is hidden from the user

HTTP's the most important aspect of Nodejs

HTTP is a first class citizen in Node, designed with streaming and low latency in mind. This makes Node well suited for the foundation of a web library or framework.

Note - Just because Node is designed without threads, that doesn't mean one can't take
advantage of multiple core  in environment. Child processes can be spawned by using our child_process.fork() API, and are designed to be easy to communicate with. Built upon that same interface is the cluster module, which allows you to share sockets between processes
to enable load balancing over your cores.