I don't think GenZ and Millennials have the expendable income. Between college debt and a house mortgage the extra money just isn't there, even if they find the Harley's attractive.
Harley Davidson also didn't modernise its brand. Also a lot of the image of Harley Davidson is attached to the heyday of motorcycle gangs, old movies between the 50s-80s e.g. "Easy Rider", "Electra Glide in Blue" and Music from that era.