There are a number of ways in which this has happened. Let us look at two of the most important.
First, the presidency has gotten stronger over time as the United States has experienced crises such as wars and the Great Depression. In general, the American people have wanted strong leadership during times of crisis. When the nation faces a grave danger such as a war, the Great Depression, or the 9/11 terrorist attacks, people want to know that there is someone who is going to take charge and make everything come out well. Congress cannot fulfill this role since there are too many people in Congress and so the president naturally becomes the person who we see as our leader. When crises happen, we are much more willing to let the president have power. When the crises pass, the power generally remains with the president. Thus, presidential powers have built up over time as crises have occurred.
Second, the presidency has gotten stronger as the mass media has given the president a greater ability to communicate with the people. President Theodore Roosevelt called the presidency a “bully pulpit,” meaning that the office gave him a really great opportunity to preach to the people on whatever issue he wanted. The reason for this is that the president is the only person (other than the vice president) elected by the whole country. Everyone knows who the president is and people are more likely to pay attention to that person than to any other governmental leader. In the 20th century, the spread of media like radio and television expanded the president’s ability to talk to the people, thus making the bully pulpit even stronger. This has helped make the presidency more powerful.
Over time, then, the accretion of power taken in various crises, along with the increased power of the “bully pulpit,” have made the presidency more powerful in our system.