Because technology lovers jump ship quicker than a pack of rats. Fact.
No one wants HD DVD's now. Apparently Blu-Ray won? No, actually I won. I have an Xbox 360, which means that I can get an HD/High Definition player for £20 in the shape of the Xbox 360 HD Drive.
Due to no one wanting these technological corpses in their homes you can now buy "The Definition of Perfect" at bargain bin prices. Often these hi-def discs are cheaper than their DVD predecessors. At worst, you can still get 2-5 of these films for the price of one Blu-Ray (you might need to shop around. Combined; amazon, hmv and play all offer some good deals).
It's true that they wont be producing any more films on HD, but there's an absolute wealth of HD films out there at the moment including many sci fi, action, horror and comedy classics. Plus when the well begins to dry up you can use your HD Hard Drive to play your old DVDs at near HD quality.
If anyone is interested, I've compiled a list of notable titles available in HD below:
P.S If I'm honest though, I am still quite happy with VHS...