The reason of this is that there are no official Windows 10 drivers for this Ethernet adapter. So It will be necessary to wait until Windows 10 becomes an official Operating System.
Thank you for the post. In case you are referring Intel PRO 1000 MT Server adapter, this model does not have Intel driver for Windows* 10. You may check in the download site below: (Please correct me if this is not the model you used)
I assumed the Windows* 10 drive you used is built-in driver from Microsoft* that is why when you tried updating from Intel website, you were told the driver is already updated.
Have you tried testing the PRO 1000MT in another operating system (e.g windows 7 or 8) and getting the same result?