In the beginning ...
There is one nagging fact that buttresses my claim. In the history of personal computing, with few exceptions, whatever is the most popular software in a particular category is also the most successfully exploited software. When Apple computers were the most popular computers, as was the case in the early 1980s, they had the most malware. In fact, the first PC virus, the 1982 Elk Cloner, was a Mac virus. When I got into the field of anti-virus research, the only viruses around were Mac viruses.
The IBM PC came in October 1981, but didn't become the dominant PC platform until 1986 -- not coincidentally, the year of the first IBM-compatible computer virus, Pakistani Brain. From that year until now, Windows has dominated the personal computer world, and so have attacks against it. (Check out some PC market share history figures.)
One Mac defender countered my corollary by noting that between 1983 and 1985, when Commodore and Atari computers briefly outranked Apple, and just before the IBM-compatible machines took over, Apple still had the most malware. This is true and it's a small exception to the rule, but it's notable because Commodore and Ataris were mostly home and gaming computers. Most models didn't include floppy drives (it was mostly ROM memory and tape cassette storage if you were lucky), modems, and other input mechanisms needed to spread malicious infections. Meanwhile, Apple computer owners were raging on dial-up BBSes (bulletin board systems) and trading software as fast as they could, a perfect environment for the spreading of malware.
Because I don't want to get mired in a 20-plus-year debate, I'll agree to modify my corollary to begin in 1986. Certainly something that has largely held true over the last 23 years is still trendworthy. Some readers may say that since 1986, Microsoft Windows has been the most popular software, so my claim is self-supporting to my goal. Yes, that's true on the OS level, but my rule applies to more than just operating systems.
Not just Microsoft or Windows
Find me any software product that is the most popular product in its category, and I can assure you it is more successfully exploited than its next popular counterpart. I can't think of an exception.
Windows is attacked more than its competitors. Internet Explorer is the most attacked browser. Microsoft Office is the most attacked word processing and spreadsheet software. ActiveX is exploited more than Java. The most popular software versions even hold true within a particular product family. For example, Microsoft XLS and DOC files are exploited more than XLSX and DOCX files.