As if Microsoft had to be blamed for that.
Let's look at some numbers:
There are 1.5 billion Windows devices in the market, according to Microsoft;
Microsoft intends to have over 1 billion users of Windows 10 at least;
Microsoft Office is used by 1.2 billion people worldwide, according to Microsoft;
Office for iOS and Android have been downloaded over 100 million times;
Apple had sold 122 million Macs overall as of 2012;
Apple had sold 78 million "modern Macs" according to Macworld, as of 2013;
*More than 300 million computers are sold every year, and 15-20 million of those are Macs (the most Apple has every sold I guess were 19 million in 2014).
Mac numbers are nearly irrelevant compared to Windows numbers. Of course Microsoft will make a better Office for Windows. It's where the market is. Any huge investment on Mac Office may simple not pay off, while huge investments in Office for Windows is a requirement given this very attractive market for competitors (such as Kingsoft, for instance).