Why the default desktop environment of most popular linux distros is gnome? KDE seems to me an equally good desktop environment. Is it for licence reason? historic? or else?
Back in the mid 1990s Matthias Ettrich became interested in Linux. (Matthias is also well known for starting the LyX project). He was concerned about usability issues, as in ordinary people being able to use Linux, which back then was mostly for highly technical types, hackers and so forth. He happened to come across the Qt toolkit, created by Trolltech. This toolkit was proprietary, but apparently Matthias did not consider that to be a sufficiently important drawback. He was what one might term as belonging to the 'pragmatic' wing of the free software community. At around that time he started the KDE project based on the Qt toolkit. If you look at the original announcement (courtesy of Wikipedia's KDE page), you will see that Matthias referred to the Kool Desktop Environment. You don't hear about Kool any more. :-) I guess everyone is too embarrassed by it.
Anyway, what one might term the 'purist' wing of the free software community, notably including one Richard Stallman and his Free Software Foundation, were alarmed by this turn of events. So the competing GNOME project was started, whose original leader was Miguel De Icaza, who happens to be on this site. Miguel was right in the middle of all this, so he'd be the ideal person for a history lesson. The new GNOME project used a toolkit called GTK (Gimp Tool Kit) which had been created for the GIMP by Kimball and Mattis around the same time (the GIMP project was started around 1995).
Then Trolltech started feeling the pressure, and switched to the Q Public License (QPL) in 1998, and finally added the GPL as an alternative in 2000. By then GNOME had a lot of momentum, and the world had two free desktop projects instead of one.
Now, Red Hat, who then as now was one of the market leaders, was and is as concerned about software freedom as the FSF, though I gather for different reasons. So, they stuck to shipping GNOME. Debian, of course, also went with GNOME. (This was in the days before Ubuntu, which was first released in 2004). So even today, Debian and Ubuntu default to GNOME. Some other distributions chose to go with KDE, notably SUSE. I remember switching from Red Hat 5.2 (I think) to SUSE 6.4 in August 1999, and being blown away by the beauty of KDE 1. And SUSE is more closely identified with KDE, and Red Hat is more closely identified with GNOME, even today.
There was a lot of uncertainty regarding licensing of the Qt library (on which KDE is built) back when most distros were choosing between KDE and GNOME. That isn't a problem any more, but by the time it was cleared up most distros had already chosen, and this is the sort of thing that they aren't comfortable switching up without a really good reason.
Most distros tend to like to focus on the most basic of end-users that can't do anything for themselves. The gnome philosophy tends to be "it works this way and only this way" and leaves the users in a common configuration you can help them through and offers a clean and consistent interface.
KDE, on the other hand, is extremely powerful and extremely flexible. You can configure the heck out of it and tailor to your specific needs. My KDE setup is extremely customized and probably doesn't match anyone else's. Because of this, though, a user could very easily get into trouble if they don't know what they're doing. So it's likely considered "less safe".
Some of it also comes from very old decisions. Back in the KDE days when it was unclear what was going to happen with the Qt licensing options, some distros went with Gnome. Now that Qt was safe to use and redistribute under a *GPL distro, if the decision was restarted it might change the outcome. But most distros wouldn't want to "make a switch" once a critical decision was made, as it would confuse people (even if it only affected new users on a machine).
/me sits back and waits for his reputation to go down for this answer. But I'm calling it like I see it.
I think everyone here misses the point. When linux was finally starting to come into wider acceptance, and distros like Ubuntu came around, machines weren't all that powerful. The consensus back then (2003?) was that KDE was bloatware because apparently the community felt it was larger, slower, and took up more resources than they felt the DE should.
However, I think most of the critics of KDE were the type of users that would use something like fluxbox, or
awesome, claiming that KDE looked like a windows clone, and all they need is a terminal window. So with only detractors from KDE who whouldn't be using gnome anyway, Ubuntu went with gnome. And that's the only reason (imo) gnome has stuck around. I don't have statistics (although I'd love to see them), but I'd bet that the vast majority of gnome users are ubuntu users. And I mean that in a statistical sense. More people who DON'T use ubuntu as a distro use KDE as a manager.
Those criticisms may have been just back then (I have no authority to say. I'm a relatively linux DE newbie. My first full time-linux desktop was in 2004 and I got berated by my teacher (In a joking manner of course) that friends don't let friends use gnome, so I switched to KDE.)
Even before gnome turned evil (left handed window buttons? Come on, seriously? Unity? Ugh), KDE is a MUCH more pleasant working environment.
I think Gnome is more universal, when KDE is more for desktops. Besides, I think that if we said one day 'we do a FREE DE' we should stick with it. Everyone remembers switching from OpenOffice to LibreOffice. There's been a reason for that.
I think that guys from KDE teem do amazing job. But can you imagine both teems starting to collaborate and giving us a platform with shells for desk and mobiles/tablets/notebooks with awesome libraries underneath ;)