The "practice" fell out of favor as you say because laws were passed which penalized monopoliesoutlawed them and made some of their predatory behaviors very costly even existentially threateningcreated an enforcement arm for the Federal government to continuefind them and prosecute them.
The Sherman Antitrust Act of 1890 made it illegal to:
outlaws all combinations that restrain trade between states or with foreign nations.
any agreement to fix prices
Sherman Antitrust Act (1890)limit industrial output
Clayton Antitrust Act (1914) - Clarified and refined SAAshare markets
Federal Trade Commission (1914) - enforcement agencyexclude competition
.
illegal all attempts to monopolize any part of trade or commerce in the United States
outlaws all combinations that restrain trade between states or with foreign nations.Enforceable by the Department of Justice through litigation in the federal courts
any agreement to fix pricesprivate parties injured by violations are permitted to sue for triple the amount of damages done them.
limit industrial outputSelling product at a loss to put competitors out of business was outlawed.
share marketsCoordinating between would be competitors on prices was outlawed.
exclude competitionMinimum wage and safety standards also took away some of the lucrative benefits of being a monopoly and dictating such to the workforce.
selling products at a lossMonopolies were identified in court and then courts broke them up.
illegal all attempts to monopolize any part of trade or commerce in the United StatesStandard Oil
Enforceable by the Department of Justice through litigation in the federal courtsAlcoa
private parties injured by violations are permitted to sue for triple the amount of damages done them.
As demonstrated be low businesses still operate as trusts and monopolies today, this is due to a failure to prosecute, failure to take action when prosecuted and a general change of attitudes over the harmfulness of Monopolies. The argument in favor of Monopolies makes note that when Rockefeller's standard oil was broken up into 34 companies. Rockefeller's 90% ownership in Standard Oil then became 90% ownership in 34 competing companies; and Rockefeller made a lot more money than he did previously. Thus the argument goes that predatory monopolies ultimately harm themselves and thus the market itself can take care of the excesses.
What makes them inferior vehicles for commerce is from the perspective of the greater economy. Trusts are anti competitive, produce expensive products, and ultimately stifle innovation. They stifle innovation in their sector of the economy as trusts by definition don't compete for market share but dictate to the market. They stifle innovation across the greater economy as the entire economy is forced to pay large overhead and incur greater expenses due to higher costs from the trust (higher transport costs, steel costs, software costs etc). Stated another way monopolies or trusts inhibit the entire economy by taking power out of the hands of consumers along with the incentive to compete and improve ones production for the trust.
**Note:**
As demonstrated businesses still operate as trusts and monopolies today, this is due to a failure to prosecute, failure to take action when prosecuted and a general change of attitudes over the harmfulness of Monopolies. The argument in favor of Monopolies makes note that when Rockefeller's standard oil was broken up into 34 companies. Rockefeller's 90% ownership in Standard Oil then became 90% ownership in 34 competing companies; and Rockefeller made a lot more money than he did previously. Thus the argument goes that predatory monopolies ultimately harm themselves and thus the market itself can take care of the excesses.