It's mostly due to the fact that consumer camera transports can't go any faster without schredding themselves, and because DV over 1394 is isochronous, and locked into 1x. DV is 25 Mbps, Firewire itself can handle many many times that (400 Mbps?), so bandwidth is not an issue. PCs (CPU, bus, & disk) can also handle higher bandwidths. I would love to see DV become asynchronous (more like a disk transfer) which would not only allow faster than 1x, but would also eliminate dropped frames.
///d@

Sonic.....
Sony had a 4x transfer system (DV to computer), a couple of years ago, in an edit system they were pushing.

How did they do it?

I'm not familiar with the Sony 4x system, but surely it didn't use camcorders, probably decks. What's old is always new again, just give it time.

I know some people who skip the capture step altogether by using a hard disk recorder during shooting, which then mounts as a file system with AVI files, ready to edit. Swap out the HD for each project, and archive the whole project on the drive it captured to. It's the future. Did you hear about the Panasonic camera with no moving media -- it takes a small deck of memory cards.

///d@

I've never seen consumer DV equipment at anything but 1x speed.

True, the Sony DSR-85/DSR-85P DVCAM deck can playback DVCAM tapes at 4x speed through a SDTI (QSDI) interface, which is Sony proprietary, not the more standard pro-video SDI interface. The heads rotate at 18k RPM and Sony recommends their special DVCAM tape for high-speed use. This deck doesn't feature firewire.

http://www.sonyusadvcam.com/Graphics/Content/NewDVCAMcatalog.pdf

You can buy a DSR-85 from B&H. It only costs $18,715.00

http://www.bhphotovideo.com/product/131559/SODSR85/REG/2129