I think you nailed it.....the impression among collectors is the art in those early-mid 70s horror books is superior to the later issues. Sure the stories could be comparable but frankly, the art is where it's at in the current market.
Charltons have the same perception problem, right or wrong.
Hey...I'd be more than happy if all horror, to include 80s-85, became more collectible. The genre overall deserves it in my opinion as it's consistantly been better than it's superhero cousin. I don't think any rapid price rise is going to happen anytime soon though....
Jim