> or they had other requirements that necessitated microservices

Scale

Both in people, and in "how do we make this service handle the load". A monolith is easy if you have few developers and not a lot of load.

With more developers it gets hard as they start affecting eachother across this monolith.

With more load it gets difficult as the usage profile of a backend server becomes very varied and performance issues hard to even find. What looks like a performance loss in one area might just be another unrelated part of the monolith eating your resources,