They want people on the latest stuff so they can try and sell them apps from the Windows Store. They get a 30% cut on all the sales and it's worth billions to them. They'll still make system makers, OEM's pay for a license and business but end user consumers who upgrade from older versions are the laggards that they make no money from if they never upgrade.
Just very good business decision. Personally I'll never use the Windows Store but I'll take the free OS upgrade.