Junior developers are terrified of their application running slow, and not having the technical aptitude required to debug and fix the performance problem.

Additionally, Junior developers do not care about data integrity (the customer's problem), or operational complexity (the SRE's problem).

Finally, Junior developers consider software to be much like bread. Older software is bad (such as SQL relational databases), and newer software is good (NoSQL).

Combine all three of these flawed systems of thinking, and you have MongoDB adaption.

... also before Kafka, the defacto queue broker was ActiveMQ, which was a pain to deploy, so many people used Mongo as a queue.