An important early impetus for research at AT&T was the immense labor component of contemporary switching technology. Hiring the number of qualified operators needed to scale out to a fully national phone system wasn't clearly possible, let alone sound business.
The scale of the challenge over time is described pretty well here:
AT&T thus had both a clear long-term goal, national phone service, and multiple long-term research challenges needed to make that goal possible (amplification)
and economically feasible (switching).
For folks interested in delving further into Labs history, this recent paper has a wealth of relevant citations, while also clarifying some important corporate cultural framing that might otherwise be confusing: