It's probably both. Additional exploitation of targets was typically required to reach data of interest, and at a certain point you do get resource constrained having to deal with a high number of targets. It takes time to exploit them, it takes time to ingest and parse the discovered data, and even basic things like managing all the resulting shells takes time.

Based on this, as you say there were likely a high number of targets who probably were not worth the time to exploit, especially when every exploitation action increases the risk of detection and cessation of the op.