The main reason is because viruses are made for masses and since Microsoft Windows has most of the operating system market share, it qualifies to be the best operating system to be hit. Same is the case with Microsoft Office and Microsoft Outlook.
Linux is a free and open source operating system with a file system that is least friendly for the viruses. Migrating to Linux along with some other free and open source software can help you save a huge amount of licensing cost and keep your IT environment safe from viruses and hackers.
10 Reasons Why You Should Switch To Linux