Question

What were industrial leaders (Like Rockefeller in Oil Industry and Carnegie in Steel Industry) able to do by creating monopolies and establishing trusts?

Answer 1

Industrial leaders earned huge profits and accumulated large fortunes by using their market power to fix prices (or establish production quotas with the same objective) so that the final price charged is higher than the price level that would have been reached in a perfectly competitive environment (hence harming consumers that pay more for their products).

Moreover, these giant corporations blocked the entrace of competitors in their industries, so that each managed to control the whole market share by itself (monopoly), or together with a group of partner companies (oligopolistic market with trust).

Answer 2

make vast amounts of wealth by limiting competition, cutting production costs, and controlling prices.