Considering the huge progress that was made in the 90'ties on Linux and the lack of progress done after OS X came, I think it is safe to assume that OS X played a major role here.

Software needs resources to get created and lots of it. As a company or as an individual, you can't sink time and resources into software that isn't at least popular. You can try to invest in something, but sooner or later resources dry out, priorities change, etc, etc...

Desktop Linux seems to me that it was created by stitching obsolete software and quick hacks together with glue and spit. Even broken as it is, I still marvel at how functional it is for me. And even unpopular as it is, there are some normal people using it, which goes to show that it isn't totally broken or insane.

    broken or badly designed software for which 
    you have the source is more of a prison to the user 
    than a well-made, closed source one.

I don't agree there - having the source is a huge advantage, even if you aren't capable of modifying it. Just as with cars, you don't necessarily have to go to the parent company and you don't have to fix it yourself. You can always choose a local shop for repairing and tunning.

    community development simply doesn't scale

I wonder why are you saying that, when server-side Linux and related software is such a huge hit.

You can also point out to some desktop software that is free software, that is sponsored and yet community driven and that is usable. That's Firefox and it's the reason why we came out eventually from the dark-ages of IExplorer's domination.