Buell motorcycles were a fad. They created a "rice rocket" style, larger displacement bike for the street, which was new at the time for an American manufacturer.
They used an existing powertrain in production, & never came up with their own design for most of their time in business.
Kinda like the "new" Indian did before Polaris took over. The majority of the motorcycle buying public never fully embraced the mark, and without robust sales #'s ultimately they went under.
Even a fresh infusion of cash after leaving HD couldn't save them.
Too bad, as they were very innovative in their design at the time...