We can't imagine there being a good enough reason why you don't own Mass Effect 2 yet but, despite that bleak and joyless world you're living in, now's the chance to right the wrongs. Amazon has both the Xbox 360 ($40) and the PC ($30) versions on sale and, in the case of the latter, even the special edition ($38) has had its price reduced. It's like probing a planet and finding a whole bunch of Element Zero!
... oh, you don't get that reference? Well, if you owned a certain game, you'd probably know what we're talking about. Just saying!
Source - Xbox 360
Source - PC
Source - PC (special edition)