We know that English people have different accents. People from Britain have a different accent, while those from US speak in an entirely different manner. So, what is the reason behind it? Well, the first part is that after the civil war, the urban boom enveloped major part of the US. These places were populated by the Scottish majority.
Slowly, people speaking in an entirely different dialect surrounded Americans. Over time, they lost their British accent due to lack of communication with Britain and developed an accent of their own. We know Americans by their own style of speaking and many can be easily recognized.
After the civil war and the industrialization, the economic power and the political power went from the port cities into the manufacturing hubs. These included areas such as New York, Pittsburgh, Philadelphia, Chicago, Detroit, etc. In these places, the British people had a lesser influence and was highly populated by the scottish people and other settlers belonging to the Northern britain.
As a result of this, the Americans had a great influence from the scottish and rhotic english was spoken here. Hence, Americans slowly started losing the british accent. Though this was not the only reason, it could probably be a major one.