Windows 10 adoption is doing well, but Microsoft wants it to do better - they have a stated goal of Windows 10 on one billion devices a couple of years after the initial release. Devices, not just PCs. One assumes Microsoft has some number in mind where Windows 10 makes financial sense for them, and they want to exceed that number quickly.
Although they are "giving away" Windows 10 to most current Windows home users, they are poised to make money from new revenue streams through Windows 10, such as the SaaS (Software As A Service) model for things like Office, and from data mining and advertising revenues.
I believe this is the main reason that Microsoft is being so aggressive in prodding people to upgrade, and why I am pretty sure the free upgrade offer will be extended in 2016.