To put things in perspective there are 700 million PCs with Windows 7 out there yet only 4 million upgraded. That's much less than 1%
Keep also in mind that for once the upgrade is actually affordable at $39 or $15 if you bought a new PC. I could understand low upgrade numbers when the upgrades were grossly overpriced (the Windows 7 upgrade costs $119) but I can't understand it when the upgrade is 3 times cheaper.
There are probably 50 to 100 million PCs that are eligible for the $15 upgrade, but even at that price it does not sell well. Let's be real, if people are not willing to spend $15 on an OS upgrade either they don't care about the OS they are running (bad for Microsoft) or they don't want Windows 8 (even worse for Microsoft). If I was running Vista and Microsoft offered an upgrade to Windows 7 for $39 I would jump on the occasion, here I am not.
I strongly suspect that a lot of those 4 million upgrades are from Windows fans that don't mind that half the OS is actually an advertisement for the Windows phone and surface interface, not from the general public.
Maybe the low price of upgrades this time round is because MS knows that the OS will be unpopular with the masses and that no-one would pay the price they usually ask for upgrades?