The company that makes that bike went out of business. Not that much competition.
There isn't any ONE reason.
HD may have over esitimated sales.
Other bike manufactureres; metrics, new entries such as Victory, Boss Hoss, custom or custom like bikes with bigger engines, new interest in bobbers etc.
Alternative demand for disposable income.
Could be generational or fad or as mentioned above interest flowing in other directions.
Legacy impression of unreliability from decades of unreliability. I don't know if anyone in the know would say a new HD is not reliable as any other manufacturer. One couldn't write that ten years ago.
They are relatively expensive.
2005 bikes are pretty ugly.
I am sure there are other many reasons.