Because it's excruciatingly slow. I tried the workaround that was referred to in one of the front page posts, and it's really not all that pretty. It's now backing up 34 GB at roughly an MB per second. That's a quarter of an hour per GB, on an 802.11n connection. It will take me over 8 hours for the backup to finish. I'm on a late 2006 iMac (with 802.11n) and have the disk hooked up to an Extreme N. Network Utility claims a connection speed of 130 Mbps. Now, it could very well be that Time Machine isn't built for this purpose, this being a workaround and all. I'm not technical at all, so for all I know things could be optimized. But if this is an indication of the current state of things, I feel sorry for everyone who got themselves a Time Capsule and have hundreds of GB to back up.