Lack of drivers, typically. I tried installing W7 on a 2018 motherboard at one point and it doesn't ship with new enough USB drivers to be compatible, so you bo

Lack of drivers, typically. I tried installing W7 on a 2018 motherboard at one point and it doesn't ship with new enough USB drivers to be compatible, so you boot into something without mouse and keyboard or remote access of any kind, so you cannot even install the required drivers because there's no way to interact with the system at all. That said, I'm sure some of you maniacs would've found a way, an old OS image like that has to have some usable zero days.