Bell Labs invented the transistor because the low hanging fruit of vacuum tubes had dried up. Some low hanging fruit is always dried up. That never stopped progress.
There wasn't a single thing that made Bell Labs work, it was a combination of factors, of which the most important where reliable funding, job stability, and technical corporate leadership. As a government regulated utility, Bell Systems had very stable revenue and profit, so they could plan R&D activities for 20 years ahead, and they did. They hired the best graduates directly from university campuses and offered them a credible prospect of retiring from Bell 40 years later. They had no MBAs in the upper echelons. Haakon Romnes and John deButts, who were CEOs through the 60s and 70s, were both electrical engineers. The people immediately under them were also engineers. When a CEO retired, they simply promoted one of these business people with technical backgrounds, who had been with the company for several decades. By contrast, Jonh Stankey, the current CEO of AT&T has a BBA and an MBA,. He probably doesn't understand how electricity works.