Why might a monopoly or a trust in a certain industry be bad for both customers and workers?

Skilled workers in such an industry have to work for the trust/monopoly or not work in their skill set. This gives the employer much leverage in pay/benefit negotiations.

Since consumers have no choice but to buy from the trust/monopoly they have to pay (usually) higher prices than they would if there were competition between businesses owned by different people.

Without competition there might be little incentive to improve product quality and to develop new products.

new evidence is discovered.

oversights and errors are corrected,

new technology is applied to evidence.