Those trusts arose at a time where there was very little regulation. There were tariffs protecting the industry as a whole from foreign competition, but that wouldn't have affected the rise of domestic competitors.
Foreign competition is important, because some things (like oil drilling or sugar farming) are only doable in a few places, so if one company captures those places it'll be tough to compete.
Also, the fact that some companies became very dominant doesn't mean ipso facto that they harmed consumers. If a company becomes huge fair and square (as opposed to via regulatory capture or other coercive means), it may just mean that people like their product the best.
Also, the fact that some companies became very dominant doesn't mean ipso facto that they harmed consumers. If a company becomes huge fair and square (as opposed to via regulatory capture or other coercive means), it may just mean that people like their product the best.