...you might ask instead why we're still waiting for the Year of the Linux Desktop...
One big reason for that is that the meaning "the year of the Linux desktop" has changed. Back when the expression was coined, it meant the year when Linux was good enough for the average computer user to use it as their OS without needing regular intervention from a tech support guru just to keep it working and up to date. From my POV, that came almost a quarter of a century ago, but by that time, it had mutated to mean the year that Linux overtook Windows as a desktop OS. Will that ever happen? I can't say, but I'd think that the more MS treats their customers as a resource to be exploited he more likely it is to happen. I do know that I've migrated two middle aged women to Xubuntu at their request and they've been very happy with it ever since. I also showed a high school girl running Linux Mint what Compiz can do, and then told her how to install it. Once she knew that it was available from her software store, she was able to install it, have it run at login and configure it without asking one more question. I'd say that Linux is ready and more than ready for the average (bear) user to use as their Daily Driver and as far as market share, I'd think that the fact that its percentage of desktop use speaks for itself.