Of course, these companies couldn't exist before the industrial revolution, but the late 20th Century also seems to have been a curiously bad time for them. There are specific examples of business decisions that were all wrong in retrospect (e.g., Kodak) but what about the larger business/industrial phenomenon?
Bell labs is still doing major innovation work as the research and development arm of Nokia. They are responsible for most of the patents surrounding 5G implementation and are leading the way in terms of network slicing and edge cloud development.
This will be a giant oversimplification, but there’s a ton of great articles and books on the subject:
The biggest factor in (specifically) Bell’s dip in early 1980s was the adoption of anti-trust laws by the U.S. AT&T was a giant monopoly that basically flowed unlimited amounts of cash to Bell Labs to work on communications challenges. With divestiture/monopoly breakups, the clear mission for research —and the money — went away. This forced Bell Labs to find a new parent, but none would compare in scope and ability to AT&T, though they seem to have found a home with Nokia.
Business models also changed — startup culture and modern corporate culture don’t allow for 10 year research horizons, i.e. they can’t wait years for research to yield results when you have shareholders and VCs to answer to each quarter.