I think it's simple.... Computer sales are bad across the board for the most part. Their lasting longer and it's just not needed to upgrade a computer all the time. Plus, I think many customers are moving to mobile computing which again is slowing desktop and laptop sales.
Apple needs to continue it's focus on iPhones and iPads to win and they need to give their current customers a good reason to upgrade their workstations and laptops... maybe that will come soon?
I think so too. For a majority of the market that Apple caters to (and most of them do NOT visit this site), there's just no "killer" feature that would make them want to upgrade. Upgrading to a new computer every year is also much more expensive than upgrading a smartphone every year too, and since Apple products last longer than their PC counterparts as a whole for the normal user, it makes sense that Mac sales are down. People who have just been introduced to the Apple ecosystem within the last few years with the iPhone have already purchased Macs most likely. All of this coupled with the fact that tablets are getting more versatile and powerful. I'm sure PC sales are not so different either.