Good question
Of course the limited production and unique attributes will help, but you must look at the social/environmental/economical aspects of the collector car industry (both past and present) as well as the physical uniqueness.
For example ... The late mid to late 60's were the high point (social) of the muscle car era ... before the regulatory (emmisions) and the economical (fuel). The "kids" at that time ... dreamed of owning one of those cars. Of course, life happens, kids are born, bills need to be paid ... so those "kids" did not have the disposable cash. Then/when ... as those "kids" become empty nesters ... or get to the point in their lives that they have disposable income ... the dreams start to come true. The market comes alive and the unique, rare cars become the most sought after.
My prediction ... the next generation of "kids" who had dreamed of owning the 240/260z when they were new ... will/are coming of age. Of course the current economic downturn has impacted the entire market ... it will not last forever.
Now think for a minute ... what early-mid 70's cars stand out in your mind as game changers in the auto industry ...
Which cars started the japenese invasion into the sports car market ... while surviving the economic times
Which cars had the lines/style that is timeless
I have many other reasons ... but I would be writting a book ... lets avoid that!