It's a simple statistic: the Linux kernel is currently, by a large margin, most common foundation for FOSS operating system distributions. That kind of unification is something that I could scarcely have imagined fifteen years ago, and I'm completely in favor of it.
There is, however, a darker side to the success of the various GNU/Linux distributions (to say nothing of Android): quite a few software authors and hardware vendors have fallen under the enchantment that supporting the Linux kernel and the GNU userland, to the exclusion of other platforms, constitutes fully embracing open-source software. I'd like to assert the potentially dangerous opinion- and I'm considering buying a set of full-body armor for this- that it doesn't. In fact, I'm going to go so far as to suggest that supporting only a single operating system, even one both as open and as popular as GNU/Linux, to the exclusion of others is antithetical to the spirit (though perhaps not the definition) of "free and open-source software."
How can I say that, when something that is both... well... free and open source... is the very definition of the term? Because, in my twenty years of dealing with FOSS, there has always been an implicit understanding that "free and open-source" also generally implied "cross-platform." That was certainly not ubiquitous: there were always a few FOSS applications that were just for Windows, or just for Mac OS (X and 'classic'). The bulk of FOSS software, and reliably the most popular tools, were generally made cross-platform. They might start out as Linux-only, or Windows-only, or Mac OS-only, but were ported within short order.
The graphical terminal client PuTTY is a great example of the above model: it started out as a Windows-only program, and it filled a niche there which was otherwise unoccupied (i.e. free graphical SSH client and related tools). Later, as Linux gained more mainstream popularity, many users coming from Windows were intimidated by the different-but-perfectly-capable OpenSSH clients in the terminal, and wanted their familiar PuTTY GUI client. So the author of the original piece got some help, put commendable work into it, et voila! There's now a Linux port, and there are rumors of official ports underway to both Mac OS X and 'classic' (yes, you read that last one correctly).
At the risk of sounding like the quintessential old man shouting at the kids to get off of his lawn, that's how FOSS should be. The original developer(s) may or may not have time and skill to port their software to other platforms based on demand, but if others are willing to take up the mantle and do the work, more power to them!
That's not what I'm seeing in many open-source projects right now.
Reading the freedesktop.org page linked above, and the corresponding Wikipedia entry, one might be led to believe that DRM is only for Linux. Neither page has any mention whatsoever of DRM being used, available, or supported on other platforms. Unfortunately, nothing could be farther from the truth. I use 'unfortunately' here in a very encompassing sense- both the omission itself and the usage on other platforms are unfortunate.
DRM is used on Linux, all of the modern BSDs (FreeBSD, NetBSD, OpenBSD, DragonflyBSD), Solaris (since the OpenSolaris days), and probably quite a few other lesser-known FOSS operating systems besides, and probably even some otherwise closed-source 'Unix-like' systems as well.
Good luck telling anything in the above paragraph to the "official" developers; they don't want to hear it.
Oh, and by-the-by, if you're on a FOSS operating system and you have anything even remotely current from Intel or AMD in terms of graphics hardware, you'd better have DRM and KMS or you're up that proverbial and 'fragrant' brown creek, and without any of the proverbial paddles.
In short, in terms of porting, the whole kit comes together, or not at all:
That would be bad enough in an isolated incident, but it's becoming quite common. Certain authors of software for Linux have begun the duplicitous behavior of advocating the "free" part of FOSS, while simultaneously resenting or scorning the "open" part. Mercifully, it's still the exception rather than the rule, but I find even a single instance of it to be counterproductive.
How has this happened?
This, too, shall pass...
My guess is that Linux will eventually become too main-stream for the most talented developers, and there will be a shift away of the greatest talent back out to other FOSS operating systems for a while, and Linux will hit a period of stagnation and (even more severe) fragmentation. That will be the "catch-up" period for other FOSS platforms, before the pendulum eventually swings back the other direction.
We can look at history, politics, the economy... in fact, just about every element of human culture, and see oscillations in ideas and values over time. I suspect that the FOSS movement will prove itself to be no different, and that popularity will vacillate over the course of decades between the more militantly-open systems (GPL licenses, Linux, etc.) and the more permissively-open (MIT / BSD style licenses, the BSDs and OpenSolaris, etc.).
Am I right in those guesses? Only time will tell.
EDIT 1 (10 December 2015): One prominent example that I suspect is related is Sarah Sharp's "Closing a Door"