Whenever a function call is made, an execution stack is created in the stack memory to store the variables of the function. Tail call optimization basically means that you reuse the allocated stack in memory if there's no information in that stack that is required later in the code execution sequence.
When a function call is made inside another function call, a new execution stack is created for the inner function call. However, the problem is that the inner function execution stack takes up some extra memory--that is, it stores an extra address, representing where to resume the execution when this function finishes executing. Switching and creating the execution stacks also takes some additional CPU time. This problem is not noticeable when there are a couple of hundred, nested levels of calls, but it's noticeable when there are thousands or more nested levels of calls--that is, the JavaScript engines throw the RangeError: Maximum call stack...