When you don't have a monopoly, the investor mindset is that the company should be laser-focused on "core competencies" (buzzword, but important) and return excess capital to shareholders - who then provide it to other companies that will innovate in the field. Keep in mind, the universe of alternative investments go beyond the stock market/PE/VC.
Capital is tied to shareholder value. When you can't point to something creating value, there isn't a reason for capital to stay. For a company to maintain a research lab, it needs to be perceived as something other than a cost center. In contrast, Bell was able to entertain its own full-fledged R&D labs because shareholders expected them to create new avenues of profit themselves because they were the monopoly.