I can explain it in one way
Retail games will (almost) always be cheaper than downloadable equivalents, the cost to manufacture and ship the discs is fairly insignificant, and on consoles at least one company has a monopoly on all downloadable sales. As for why it's cheaper on the Iphone, it's a completely different market there - different customers and different competing games - if you want to make money on the app store you need a cheap price
So the reason for all 3 of these is that complex mathematical calculations determined that this would maximise the profit for the relevant companies.
:P
Sucks though...