I'm not sure why this is.
Is this not because as it's the lower tones that travel furthest, on ADSL it's the upstream that uses the lowest tones. Hence even on long lines, upstream is still at full kahuna, and just the downstream dies off.
On VDSL the 'blocks' of up and downstream tones are spread through the frequency range. Hence upstream dies back in a far more pronounced fashion. Have seen a fair few 40/10 lines where the downstream is at 40 but the upstream is already off the pace, say 7 meg for instance.
This is of course all conjecture on my part, so will happily sit and wait to be shot down in technical flames.