In which way are they a monopoly? Is that US specific? In the UK/EU there are at least half a dozen different tractor brands which I could name, all of them with significant market share.
I don't think a brand being fashionable, to a middle aged demographic, makes them a monopoly. They're just popular. There are plenty of other similar bikes that can be bought.
edit: I'm an idiot. I read the comment as being about Harley Davidson.