I think you're approaching this from an end-user perspective, as though Linux desktops are equivalent to products being sold to consumers, want to compete on market-share, etc. That's missing the point of what drives the Free Software ecosystem. Since people can produce their own software, they will. The desktops themselves are down-stream of different toolkits, and then set-ups for those desktops in various distros are downstream from there.
The GTK was developed for the GNU Image Manipulation Program, and then developers said "hey, we can use this to make a desktop with!" and they produced Gnome. Qt was developed, and then developers said "hey, we can use this to make a desktop with!" and they produced KDE. Others looked at GTK and said "hey! we can produce a desktop which is more lightweight than Gnome!" and developed Xfce. Since lots of people find programming fun, and they love sharing stuff, lots of stuff gets made.
This is a good thing. This isn't a competition, because this isn't a market. Individual installations aren't commodities. The only way to have a "standard" would be to go around telling everyone they're bad people for creating and installing and releasing new stuff. Just because Apple and Microsoft have end-users brainwashed into being terrified of knowing what's under the hood of their computer doesn't mean Linux has to go hide all the gory details from you.