I'd love to hear your theories. The Triumph story is pretty interesting - quite a corporate history, and yet...fast depreciation, not-Kawasaki build quality, stretched US dealer network?
Triumph's story is a great success story, no doubt.
My theory (executive summary version) has mainly to do with the nature of the "typical" (and yes, I know I'm generalizing) buyer at various price points. To start, Triumph doesn't sell any cheap beginner bikes. They cost significant money. Except for people who have a lot of money, those willing to spend a lot for a motorcycle, thus a new or barely used one, are usually more motivated, more informed. People who read magazines, websites, forums, really into it. The farther downstream you move, the more you get less informed, more casual buyers. Now I know there are exceptions, many hard-core riders who buy rat bikes cause they're cheap or that's what they love, but I'm talking in general.
Those less informed used-bike buyers shy away from Triumphs because they don't know them. They know Harleys, or they know Hondas, other Jap bikes. They may never have SEEN a Triumph dealer. Triumphs get crossed off the list for lack of familiarity and thus comfort level.
This is based on experience with friends trying to sell used Triumphs, often at killer prices, and the feedback they've gotten.
More to it than that, but I think that many guys buying a $2,000 bike or a $2,500 bike are going to shy away. Just theory.