Final answer:
The ideology surrounding U.S. antitrust policy has shifted due, in part, to the increased number of American stockholders who benefit from monopolistic firms, coupled with competition from foreign producers in the global market.
Step-by-step explanation:
The reason why American ideology regarding antitrust policy has changed over the past few decades is that more Americans are stockholders and benefit from monopolies. As more individuals invest in the stock market, there is greater public interest in the success of large corporations, which can lead to less support for antitrust actions that might reduce the profitability of these firms. Moreover, the advent of a global market has introduced competition from foreign producers which challenges domestic monopolies and potentially decreases the urgency for stringent antitrust enforcement.