Why did Microsoft do it?

For existing users of W7, nothing, but have you tried to buy a new PC or laptop without W8 installed?  Not easy to do.  Yes you can go through the hassle of "downgrading" back to W7, but for non techies that could be a daunting task.  Also if you downgrade, most PC makers will no longer support your system.

Yeah, I'd say that anyone buying a new PC or laptop is being forced to use W8.