Well, to cut a long story short, Wiesmann failed, because they made the old mistake many small carmakers make: They make the car THEY wanted to own and assumed that all others actually wanted it, too. They were chasing a dream and hoped others would magically have the same dream and buy their product in masses. Thing is, though, that people didn't.
It's the old story of the dreamer but that story has come to an end again for now.
When you want to make a car that stands out from the masses, you have to accept, that your company will never ever grow to a size, where you can be financially comfortable over a long period of time. You always have to keep the possibility of insolvency in the back of your mind and act accordingly - at least if you wanna remain independent.
Wiesmann in my opinion made the mistake to assume they were a "real" carmaker, while in reality they only served a niche market. The company grew until that niche was saturated. Now that's the case and that was it.
If they want to start over, they've got to come up with something more compatible with the masses. They might as well be bought by BMW or another carmaker, who knows? But then all the Wiesmann fans (who never bought one of their cars) will probably shout "treason" and march with forks and torches towards the Wiesmann headquarters.
Tough world...