Understand that there is a difference between GNU software and GPL'd software.
Many people think that everything GPL'd is GNU software, which is simply not true. GNU software is *only* that which is listed as part of the GNU project. A list of all GNU software can easily be seen here.
If you take a standard Linux distribution, and list out all the packages there, you will find a fair amount of GNU software, agreed.
However, you will find a *MUCH* larger number of packages that are GPL'd, but which do not appear in the list of GNU projects/software. And this list of non-GNU software is *far* longer than the GNU content.
Now how does one go about being fair?
By calling it "GNU/Linux", you are giving credit to GNU, which *is* a part contributor to the distribution, but by no stretch of imagination the *only* (or even the largest) one.
To be fair, you would have to name the distribution after *all* the contributors whose GPL'd (and not necessarily GNU) packages are included.
For example, Apache represents an extremely important reason for people using free/opensource software. As you can see here, it says "This is not a GNU package". It is not even GPL'd software! Therefore, it is a project in its own right, and like GNU, should be acknowledged.
So it should be GNU/Apache/Linux.
But what about.....
This, as we can see, quickly becomes impossible.
The term "GPL/Linux" would be more appropriate, because by *any* stretch of imagination, GPL'd code exceeds code from GNU projects.
But even that would not be fair - what about the code carrying different licenses and included in the distribution? Apache, for example, uses the Apache Software License, which is GPL compatible, and Postgres is BSD licensed!
Therefore, the closest "fair" naming convention would be "Free/Linux", but even there you run into problems, thanks to the way people try to differentiate between "Free Software" and "Open Source Software".
As you can see, there is a very good and practical reason for calling it "Linux".
While it may seem great in terms of ideology to call it GNU/Linux, it is simply unfair to the majority of contributors to the distribution to do so.
[Addendum] As pointed out by so many people, the FSF (the people behind GNU) do not recognise Linux as a GNU package, either.
http://atulchitnis.net/writings/gnulinux.php
