I think there are a couple factors helping to keep FDC prices lower than they once were.
An over-abundance of covers produced by
Canada Post, although they are usually visually very nice and a lot of work has gone into the design, stamp and cancellation, drives the price down.
Also it floods the market with more covers than there are collectors. Someone didn't sit down and look at things long term wise. They went for the quick profit of copying the private FDC makers who only produced enough covers to satisfy the market but then went with the idea that more is always better.
If the Mona Lisa was available to everyone, it would be appreciated for its beauty or effect on one but not cost a lot and not treasured as a rare item. Basic supply and demand.
Then, on top of this basic market mistake, the Post Office (and perhaps others worldwide) try to sell the items as items to be treasured by generations to come at the same time the price is dropping in the market. Using marketing strategy instead of human psychology might work if people could have an emotional bond with the item (which happens in collecting Royals items) but not as much for every single over-issued stamp and associated article.
The second factor is how they are marketed, or perhaps marketed by those who do not have the sums that large corporations do to spend on research and sales.
Think of he Great Britain maximum cards. They were a non-item until some dealers got together (or copied the smartest one amongst themselves) and started producing and marketing the cards as a great collectible item. I am not sure how they did it but I have heard it mentioned before now. All of a sudden (relatively speaking) it seemed that Maxi card collecting was a Big thing. It is quite a world wide thing, perhaps not so much here in Canada and the US (another marketing mistake I believe) but elsewhere.
I've tried to make my own but the ink used in cancelling here tends to run. I have seen one fellow sell his made ones for a lot as he said that he had to wait 3 weeks fore the ink to dry so it wouldn't smear and so people should pay for that.
It is somewhat a matter of selling skills and a matter of friendship skills (which selling has a lot of in it) and a matter of perception my the buying public of what is a Good or Cool thing.