In what ways, exactly, is Linux behind the others?
Also, productivity is highly, highly subjective. At any given time, I'm running at least Windows and Linux, sometimes I also have a Mac going. Of these, I'm most productive in Linux, the Mac, then Windows. And no, it doesn't have to do with software availability, because the software I run runs on all three platforms.
Mac OS only got big again because the iPod took off not long before they switched to Intel hardware. Those two things allowed Apple to regain a foothold in the desktop world.
Right now, one of the biggest hurdles for Linux is that it's not pre-installed on computers that are sold in retail environments. This means people aren't exposed to it, period. Did you know that nearly all Dell computers are certified Linux compatible? Yep. They are. They're made that way due to their international market, which has a rather high percentage of Linux installations. You can't get it on non-netbook machines in the US, though. Another issue is the lack of help provided with those OEM installs for how to do the basic stuff, though Windows and Mac don't really offer that, either (hence the accusations from all sides that the "other" isn't nearly as intuitive as "this" one).
A large amount of the compatibility is there. I've yet to run into compatibility issues with hardware (though AMD graphics cards are still lacking, due to the way they decided to handle drivers, but nVidia are phenomenal). On the compatibility front, it's just a couple of big software vendors that are holding things back (Adobe being the big one).
The companies who haven't just "changed the binaries to run on Linux" have found it too cumbersome and can't justify the resources necessary to do it. I suspect, though, that that will start changing as Valve and Steam on Linux gain traction and start illustrating what the market really looks like when people are actually given the option to buy a Linux version of popular software.