Why can't I make this partition active on Windows 10?

Judging by the fact that your boot drive has an EFI System Partition, I surmise that the disk is GPT and your computer boots with the UEFI specification (instead of MBR with BIOS). That boot method needs no concept of an active partition. If you tried to make a GPT disk's partition active using DiskPart, you would get this message:

The selected disk is not a fixed MBR disk.
The ACTIVE command can only be used on fixed MBR disks.

Apparently, Disk 2 - which you used with Windows 8 - is an MBR disk. Your previous system booted using the active partition, but assuming your current bootloader is on the EFI partition on Disk 1 (you can check this with bcdedit /enum /v), the absence of Disk 2 won't affect your system's ability to boot.