Well, people wanted to replace the LAMP stack (Linux, Apache, MySQL, and PHP/Python/Perl) for a very long time. When JavaScript was made possible to run on the Server side ie NodeJS, it paved way for a full development stack which is the MEAN stack.
The first advantage of the stack is the single language throughout the whole application - JavaScript. This made it easier for a person with JS knowledge to create a full on working application. This also made way for front end developers to explore more into how server side programming works.
Some startups and organizations thought that using a stack with one language will help them in limiting their resources. And easy delegation of work among the developers.
Even though MEAN stack is an awesome alternative to the traditional LAMP stack, it is still in its early stages. It is adopted only by few ambitious startups, but not by bigger organizations. They are still sticking to the legacy technologies. Also the components of the MEAN stack which is Angular, Express and Node are still in their childhood stages. There are no proven and complete use of the MEAN stack on a full scale solution.
Well organizations like Walmart etc. used Hapi.js and were successful in it, it tempts the other organizations to go forward with it , but also makes them skeptical on using it since replacing the already existing technology will be a bigger headache.
Also since a lot of the current employees are trained in Java, Python etc, developers in JavaScript are very few and JS is almost considered as an additional language to be known for a Java developer and the likes. JS hasn't been considered as a single language on its own.
Maybe few years down the line, the MEAN stack will prove itself a noteworthy alternative to the traditional stacks.
I work in the Retail Domain and a big organization. We are actually planning to use the MEAN stack for our inside projects. But no one ever gave a simple thought of using the MEAN stack in PROD.
EDIT: One more important reason these stacks are tempting is because they are all Open Source. Say MongoDB for example. Owning the software license every year is a big hit on the profits of the organizations with DB's like Oracle SQL etc. Going with an Open Source DB is a very cost effective option. But still they are skeptical on using it for Production.