For a long time offshore wind was just more expensive then onshore and the US had the space. It was only in the last decade that large scale offhsore projects started in Europe and China. That brought down costs and offshore wind turbines have become much bigger and more efficent, which makes them competitive.

The US also has the Jones Act, which only allows ships build in the US, own by Americans and staffed by Americans to transport freight between US ports. That makes building offshore wind farms very difficult as the US does not build those ships and lacks qualified personal.