The business perspective

From a business point of view, using different languages for the front- and back-end means they need to hire two different types of developer: JavaScript developers for the front-end and, say, Python developers for the back-end. If there's a large backlog in back-end tasks, front-end developers wouldn't be able to help (unless they also know Python). This makes resource allocation more difficult for project managers. But if everyone develops in JavaScript, then this problem becomes null.

Furthermore, using JavaScript for the entire stack makes the development process more efficient. Apart from the efficiencies gained by avoiding context switching, a single developer can now develop an entire feature from start to finish, as they can code both the front- and back-end.