Its hard to say because they're not explicit about this but, despite being a decade-long Mongo apologist myself, I'd totally believe that they liked the linear scale story for Cassandra more from an infrastructure/config perspective.
Increasing top-end write throughput or replication in Cassandra is just adding more nodes, where in Mongo its not just adding nodes, its adding replica sets (which consist of 3 or more nodes). So there's a few more layers of complexity to that story. You need more replica sets to increase write throughput and need more nodes in replica sets to increase replication.
Im hand waving some details here, but I've worked with both platforms can definitely understand the choice at least from a pure infra lens.
Increasing top-end write throughput or replication in Cassandra is just adding more nodes, where in Mongo its not just adding nodes, its adding replica sets (which consist of 3 or more nodes). So there's a few more layers of complexity to that story. You need more replica sets to increase write throughput and need more nodes in replica sets to increase replication.
Im hand waving some details here, but I've worked with both platforms can definitely understand the choice at least from a pure infra lens.