There are many reasons for the Mongo DB getting famous for the development of web applications. Though mongo DB is not an appropriate tool or I should say far from it. It has a lot of challenges and yes not fit for all types of web applications. But at the same time, Mongo DB works best at the time of web applications for the following reasons:-
1) The database of Mongo DB has the ability to hold a lot of documents and all the documents have their own size, content, and number.
2) This database can be used for numerous web app actions that include data management, content management, mobile applications, IoT, big data, etc.
3)The relational database is scalable. In this, the server can host the complete database.
I hope now you got my point that it has its own pros and cons.
If you wish to choose the right tech stack you can go through this article that I find.