I think the fact that they are aluminium plays a part, there is a very vocal 'Steel is Real' faction among cyclists, I count myself as a part time member of that particular tribe having owned almost exclusively steel framed bikes my whole life, and I do possess a preference for the feel of a good steel frame. I think to a lesser extent Cannondale suffers from the same thing.
Having said that a beautiful bike is a beautiful bike and I've dished out as many
to Kleins as to any other brand on here, there's no denying that Klein did make very distinctive visually pleasing frames with often astonishing finishes, and now I own one and am in the process of building up a second I've had the chance to glimpse some of the other unique characteristics these bikes possess beyond the aesthetic styling.
Could there also be an element of Klein's falling victim to the way some owners and evangelists are perceived? In the same way that certain brands of car seem to attract more than their fair share of unsavoury owners, the point being that the bikes have become associated, rightly or wrongly, with a certain type of irritating person.
I don't know enough about the subject to comment further except to say that I have read and heard people refer to Klein owners in less than complimentary ways.