Nitpick: Historically, Microsoft ported to Windows, not to the Mac. Both PowerPoint (http://en.wikipedia.org/wiki/Microsoft_PowerPoint#History: Originally designed for the Macintosh computer) and Excel (http://en.wikipedia.org/wiki/Microsoft_Excel#Early_History: Microsoft released the first version of Excel for the Macintosh on September 30, 1985, and the first Windows version was 2.05 (to synchronize with the Macintosh version 2.2) in November 1987.) were first released for the Macintosh. Word technically arrived for DOS first, but that version wasn't WYSIWYG.
Back to your question, my guess is that Microsoft realized that supporting Mac OS was feeding their competition more than it would bring in additional money. That didn't happen with the other applications because they were released years earlier (Access is from 1992; Word and Excel from 1985, and PowerPoint from 1987), when it was far from certain that Windows would win the desktop.
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Nitpick: Historically, Microsoft ported to Windows, not to the Mac. Both PowerPoint (http://en.wikipedia.org/wiki/Microsoft_PowerPoint#History: Originally designed for the Macintosh computer) and Excel (http://en.wikipedia.org/wiki/Microsoft_Excel#Early_History: Microsoft released the first version of Excel for the Macintosh on September 30, 1985, and the first Windows version was 2.05 (to synchronize with the Macintosh version 2.2) in November 1987.) were first released for the Macintosh. Word technically arrived for DOS first, but that version wasn't WYSIWYG.
Back to your question, my guess is that Microsoft realized that supporting Mac OS was feeding their competition more than it would bring in additional money. That didn't happen with the other applications because they were released years earlier (Access is from 1992; Word and Excel from 1985, and PowerPoint from 1987), when it was far from certain that Windows would win the desktop.