If you are targeting businesses, virtually nobody will run Mandrake (which does not exist anymore), Mandriva, Arch, or whatever. When you target Ubuntu LTS and Red Hat Enterprise Linux, you have probably covered most of the enterprise user base.
(Otherwise, the Linux version of a program might cost 2-10x what a Windows or Mac version will cost.)
This surprises me, because Apple changes and breaks a lot of stuff every macOS release and Apple only supports one or two versions back for security updates. In the meanwhile, RHEL only releases every half-decade or so and supports every release for much longer. Ubuntu LTSes are only released every two years and are then more or less frozen as well.
I like to think of desktop Linux as a DIY hobby; but not something that you can expect commercial software vendors to support.
Except if you care about servers. Or developers for that matter.
Sure, for everyday end users, supporting Linux can be a pain, but if we’re still talking Enterprise, there are only a few distros that would need to be covered.
I don't use desktop Linux myself and I'm not usually a desktop Linux advocate at all, but I know that a lot of developers use desktop Linux professionally for good reason. Public administration in Europe is also very gradually moving towards using more Linux on the desktop (in spite of well publicised setbacks). There is a lot of public pressure in that direction.
Desktop Linux may not be important now, but it probably has more growth potential than anything else. And if it grows even just a little bit, the impact on Windows/Mac-only cloud solutions will be disproportionate. It doesn't take a huge percentage of Linux desktops to make Windows/Mac-only cloud solutions very cumbersome.
I think it's a smart move by Dropbox to occupy that niche now, even if it's loss making. There could even be a bit of a moat, because Microsoft and Google both have reasons (Windows/Chrome OS) to drag their feet.
Thus, what happens is that so-and-so runs such-and-such which happens to be incompatible because of some whacky configuration alignment that no one considered.
The test matrix then becomes much more complicated than a traditional test matrix targeting common Windows and Mac configurations.
That's fine for expensive software where the end user might have a very close relationship with the vendor, but for "cheap" software the cost of supporting every possible way someone configured their computer is significantly higher than targeting Windows and Mac.
(Otherwise, the Linux version of a program might cost 2-10x what a Windows or Mac version will cost.)
I like to think of desktop Linux as a DIY hobby; but not something that you can expect commercial software vendors to support.