I think the main aspect is that crosstalk gets worse as the frequency of the signal increases, so for 60-80 Mbps, the highest frequencies in use will be in the downstream D3 band. So the downstream is worst affected by crosstalk because the downstream is using the highest frequencies.
There may be other factors, upstream may generally operate at a lower power output than the downstream. The downstream signals will be strongest at the cabinet, which is where all the lines are close together, although I don't know how that would affect the SNR, highest signal strength vs. highest crosstalk noise.