Im not sure of the reasons, but maybe becuase the first wave of internet shoppers have finally learned how to do it! (plus of course efforts by sites to make the ordering process simpler)
I think we underestimate the patience, knowledge, and comfort required for on-line shopping to be a credible alternative. The early attempts were designed by techies who assumed the mainstream had good knowledge of the Web - they hardly ever asked the consumer or trialed their softare with Jo Blow.
Maybe now, people have become more confortable with it, and secondly the interfaces/simplicity has improved.