Well, what do you expect people to buy? I'm sitting on a six year old MBP, having gotten rid of my 2009 iMac. New iMac? Meh, it might come to that, but I already have enough displays. Mac Mini to go with the display I already have? Surely you jest to suggest a four-year old model. Mac Pro? See: Mac Mini. Current laptops aren't an option unless I get desperate (i. e., current MBP dies tomorrow). I mean, I want a new Mac, there just isn't anything out there that makes me feel like I'd be getting an upgrade.
So my theory is that a lot of people are in the cycle I've been in for the last three years: "Meh, I'll wait to see what they have next year...<year passes>...Meh, I'll wait until...holy shit, is this thing really six years old?...<year passes>"