probably cause most people do their shopping over the weekend, and they will be posted out on monday and by the time they get it, it will be like tuesday/wednesday and then it's back to thursaday.
if it was a monday all the people that ordered over the weekend would be waiting for their stuff to be sent out and by the time it is ready to be sent out the new deal of the week will be on and a load of people would be ****** if the stuff they just bought was on offer now, and probably complain and ask if they could get the stuff on the special offer price.