The link-and-pin coupler proved unsatisfactory because:
It made a loose connection between the cars, with too much slack action.
There was no standard design, and train crews often spent hours trying to match pins and links while coupling cars.
Crew members had to go between moving cars during coupling, and were frequently injured and sometimes killed.
The links and pins were often pilfered due to their value as scrap metal, resulting in substantial replacement costs. John H. White suggests that the railroads considered this to be more important than the safety issue at the time (see reference below).
Railroads progressively began to operate trains that were heavier than the link-and-pin system could cope with.