Who cares? ALL my customers are on Windows. The tiny fragment of a market that can't run windows software is irrelevant to most people, especially those in the business of making money.
If you develop in .NET, it's not surprising that you only have customers that run Windows. It would be odd that it was otherwise.
The fragment of a market that _can't_ run Windows is very small and irrelevant.
The portion of a market that _won't_ run Windows for you app, is not small, and relevant.
Keep in mind that most servers run Linux, so business software that doesn't run on Linux is a problem.
Even the largest of those tiny minorities (Mac users) can run .Net using bootcamp or parallels or some such.
And for the rest, there's Mono, which will run a subset of .Net stuff.
That could be true if you were just talking about desktop software. Other than MS Office, desktop software is not the largest kind in business.
The real money is in business software, that needs to run (at least partially) on servers. In that case, Mac is not relevant, and Windows is not a leader. There is money to be lost if you don't target multiple platforms.
Of course, you can run a successful company, even if you don't target the whole of the available market, but there are actual, relevant, missed opportunities when you target just MS Windows.