1. Malware is a business. A criminal business, but still a business. As long as the overwhelming majority of PC's run Windows, the overwhelming majority of malware attacks will target Windows. Malware software developers write for Windows for the same reason that legitimate software developers write for Windows: That's where they get the biggest return.
2. Security and privacy are different things. While I think Windows 10 is, or can be if administered appropriately, a sufficiently secure system, it is not a private system. The integration of Cortana and other services requires the retention of tracking and usage data many find to be invasive, even if you choose all the opt-outs W10 offers.
3. AV software is typically used on Linux machines networked with Windows machines in an attempt to avoid relaying Windows viruses and malware via shared mail. It's not useful to run an AV on a Linux system that isn't on a Windows network. All the AV signatures will be for Windows viruses.
Edited by buzzrobot, 07 February 2016 - 07:05 PM.