I think the issue is that we have people who had the cars when they were new and then those people who remember them because their dad/mum/uncle/neighbour/school teacher had one but they are all getting older (myself included), now the younger generation don't really know what they are and so are less interested.
Coupled with the fact that we now live in a disposable society and people don't fix anything these days, especially something as complex as a car. I was talking in my office a few weeks back about putting some brushes in the motor of my mums washing machine and people were gobsmacked and couldn't understand why she didn't just buy a new one, the brushes where £15.
The price of classic cars has gone up and the days of being able to buy one for a few hundred quid, do a small amount of work and drive around in nice looking car are behind us, now you need a lot more money to buy a classic and it will need a lot more work because they are older.
I also think peoples expectations are higher which again increases the costs, people now want their classic to look as good as their new modern car with really good paint and panel gaps far in excess of what they were ever manufacturered to when new.
One other thing is that older cars have less/no safety features and you need to concentrate a lot more when driving them. People now are used to front wheel drive cars with ABS, DSC etc. which keeps them all out of trouble and convinces a lot of them they are Lewis Hamilton, give them an old RWD car and a wet road with no electronic stabilit aids and most of them would be in the hedge before they got to their destination.