I suppose it is kind of a supply and demand thing...
On one hand, most cars that have been kept up still have their hardtop (albeit often gathering dust in the garage.) And the cars that have not been kept up often fall to ruin, get moved around, or just sit etc. All the while the hardtop gathers dust but survives even when the car has moved on... Thus the supply outpaces demand.
On the other hand, when one of these wayward cars gets rescued -- the new owner goes hunting for a hardtop -- bringing some demand back into the equation. Often times, the guy with a hardtop to sell has no car to go with it -- and just wants it gone... And he's probably not going to make the effort to restore it first (which would make it much more valuable)... just opinin'