Because it's cheaper?
Besides, they do receive support from Microsoft with respect to DirectX and whatnot.
It's just as much of a reflection of the consumer base as it is "solidifying Microsoft's monopoly on the OS-market." It costs money to make it for Linux, and likely wouldn't result in that big of a turn around. Yeah, you might get decent Linux sales, but it's hard to say if they wouldn't have had some of those sales if it was Windows only. There is a large crossover.
I'm not sure how you can accuse them of being irresponsible. You could argue it would be irresponsible for them as a business to waste resources for negligible return.
To be perfectly frank, can you imagine how much more difficult things would be if there was an even wider assortment of basic operating systems available? Not only would a developer of PC games have to deal with different hardware issues, they'd have to deal with the nuances of different OSes. Including how those different operating systems deal with the different hardware! I suspect it'd be a mass exodus of PC gaming, as it'd become much less coss effective (than it already can be) to make a game for the PC.