My reason for choosing the MERN stack

There is obviously some more to development (design, testing, deployment, hosting...), but as far as coding, MERN covers most of the coding needs of a web application.

The common ground for each framework/technology is, of course JavaScript. Although best practices are different in each layer of the stack, a developer who is proficient with JS can easily help out in both front and back, as well as perform some operations on the database. Because all developers use the same language, code and business logic can be peer-reviewed by anyone, and teams find it easier to communicate and help each other out.

This is not a one-size-fits-all kind of solution. Some applications are clearly not suited for JS only. While front-end development is pretty much bound to JS, both the back-end and the database should be changed to what fits the project best.

Machine learning algorithms, for one, don't typically perform very well on JS. If your app relies heavily on those, or some other computation-expensive algorithms, it may be best to give other languages a go (such as Java or Python)