Why has Microsoft made it impossible to upgrade to Windows 8.1?

Long time Windows fan here. Just picked up my Surface Pro 2 and am loving Windows 8.1 ... an OS I'd really like to have on my desktop PC.

Problem is, every time I open the Marketplace there's no update. I've ran Windows Update/tried to download that one file that's supposed to make the Windows 8 update current to the point of offering Windows 8.1 as an upgrade but .. no dice.

I don't see why it's so hard to issue out a single (and pretty important) update for Windows? Why is it impossible? Am I the only one who can't seem to upgrade to 8.1 no matter what I try?