First things first NodeJS is not a language but a mere runtime environment to allow JavaScript to be run on the Server. So the main question here would become why is JavaScript not preferred for cpu intensive tasks. As pointed out earlier that since JavaScript is a Dynamically typed language which allows for flexibility but at the cost of performance as the JSVMs have to at runtime and during execution figure out such things like type of variables unlike languages like C++ in which such things are caught during compile time.
Regarding the question on why NodeJs environment is not preferred for cpu intensive tasks is that Node is designed to handle concurrent incoming connections and makes use of a concept called the Event Loop which is implemented through the C++ library libuv.
The Event Loop orchestrates the different types of incoming requests and provides the results as and when they are available. The tricky part in Javascript Programming is not to write blocking code. What this means is that the execution of code should not be halted by a task which is not related. To give you an example consider the scenario that you have an API which just reads files from the fileSystem.
Now suppose 2 users issue requests to read two different files of different sizes . There are two ways to handle this 1. Write Synchronous Code and Block the Event Loop this will result in the Event Loop to be blocked each time when reading from the fileSystem before it can address the other readfile requests although this will be some milliseconds but i hope you are getting the bigger picture. The second way to handle this is write Asynchronous Code and Not Block the Event Loop , this will keep the event loop running and what this will do is schedule these Asynchronous calls in a stack and once these calls are over they pass control back to the Event Loop to handle the output request from these calls.
And there are many ways you can do cpu intensive tasks in NodeJs. One being that you can fork new processes to handle really CPU intensive and recently Nodejs has come up with the worker threads concept which might also be useful in this case.
Hope i answered what you were looking for ... Please let me know if i was not clear on anything in my answer