Ask the kernel-power developers why KP is still based on 2.6.28 instead of 3.x! Then you'll know why. To make it short: The binary blobs needed to run the hardware are only available for the 2.6.28 ABI and since they are closed source they can't even be recompiled for newer kernels.

Or ask the Easy Debian developers why it was so long a wild mix of Lenny and Squeeze and why it still needs ugly hacks to make it Squeeze-compatible. Well, I'm an Easy Debian developer, sort of. And I can tell you that the reason is the mixed nature of the underlying Maemo system which is a Mix of Squeeze, Lenny and even older software which Easy Debian needs to be compatible with to work properly.

As for the dependency hell, have a look at the Marble thread! Then you'll see that there are incompatible versions of monav (one of Marble's dependencies) in the repos. Something like that would be unthinkable under Debian, at least for longer periods.