Granted, with an anticipated billion computers to be upgraded, many are still in the queue. There is also another side of the coin. Although Microsoft indicated that all qualified Windows 7 and Windows 8.1 computers could upgrade to Windows 10, it seems that for many devices the "qualification" is dependent upon the OEM. For example, as stated by a Dell representative here,
"As per the update from Microsoft, all Windows 7 and Windows 8.1 systems should be eligible for the Windows 10 free upgrade. But, Dell will decide which systems Dell will validate and support with Windows 10."Other OEMs may be following a similar procedure. If you are still waiting to get the upgrade, it may be beneficial to check the OEM website.
The links below may be helpful in determining whether your device has been approved by the OEM as supported for Windows 10 upgrade.
h/t: Thanks, Techie, for helping to track down the references!