Because it prevents whole classes of support issues - if you can't install it, you can't raise a ticket complaining that something doesn't work under 7 and then waste support time... even if you say "if you're running windows 7 you're on your own" people will still try it on, or ask for one little tweak to make things work... all effort that could be better spent on forward progress or real support issues. It is far easier to prevent people from being able to have issues in the first place.
Also, I agree that 7 was a fine OS, but it is more than two years in the grave... you cannot expect continued support for it. It is sad, but a statement of fact.