For many years, the Windows platform was the main target of attackers because of it being the most common desktop OS. This means that many beginner malware developers have it at home to experiment with, and many organizations are using it on desktops of non-IT personnel, for example, accountants that have access to the financial transactions, or maybe diplomats that have access to some high-profile confidential information.
With respect to this, the Mirai (future, in Japanese) malware fully deserved its notoriety as it opened a door to a new, previously largely unexplored area for malware—the Internet of Things. While it wasn't the first malware leveraging it (other botnets, such as Qbot were known a long time before), the scale of its activity clearly showed everybody how hardcoded credentials such as root/123456 on largely ignored smart devices can now represent a really serious threat when thousands of compromised appliances suddenly...