Pointed to from an answer to another question, I came across this graphic, and I'm shocked how many linux distributions currently exist.

However, it seems that most of these distributions are forks of already popular distributions with minimal changes, usually limited to themes, wallpapers and buttons. It would still seem easier to create a sub-distribution with the required changes, such as XUbuntu with XFCE4, KUbuntu with KDE4, Fluxbuntu with Fluxbox, etc.

In my mind there are a number of problems with having so many distributions - perhaps less security/stability due to smaller group of developers, and also the confusingly vast range of choice for newcomers to Linux.

Some reasons that developers might decide to fork are:

• Specializing on a particular topic (work-related topic - i.e. for a Hospital, etc)
• An exceptional architecture, that requires a special set of software
• Use of non-FOSS, proprietary technology, and such

So what other reasons are there that have caused so many people decided to create their own distributions? What are the thought processes that have led to this? And are these "valid" reasons - do we need so many distributions?

If you can back your experiences up with references that would be great.