Because hackers have become so aggressive. It's no longer just clandestine groups of anonymous people working through the internet. There are industrial scale

Because hackers have become so aggressive. It's no longer just clandestine groups of anonymous people working through the internet. There are industrial scale state-sponsored hacking efforts across the world now, actively focused on exploiting every single release of every single OS, and it's only getting worse. Also, with the wide scale adoption of broadband internet it's become possible to adopt an "evergreen" model for OS updates now, much like browsers have been doing for years.