The cost is just expensive. You have to run your HD transfer through a machine that "prints" your frames onto a digital printer... AND... because of the nature of negative and film stock, you have to re-time the master to work with the machine (changing contrast ratio, etc.).
Also, you have to digitally tell the printer exactly WHAT part of the frame you want to have printed. Like Adobe Photoshop, you can cut, paste, add mattes, zoom, etc.
It's new... and just expensive. I'm sure the costs will go down as it becomes more commonplace but, since its a fairly new technology, you just pay a lot of money for it.
Only the major studios can really afford to do this right now.
Check out this link:
Cinesite Digital Intermediate