From the perspective of someone who is a member of openSUSE and the maintainer of a non core component (enlightenment) the simple reason is there is no need as
- All window managers / software is treated equal
- openSUSE tries to be good at everything rather then focusing on one thing.
openSUSE has made it very easy through Open Build Service for anyone to package a application and get it included into the main openSUSE repository. It also has the philosophy that all window managers should be treated equally and this extends to a lot of other areas as well. The vast majority of derivatives i have seen really just slap another window manager on top of a existing distribution because that distribution doesn't support the window manager someone else wants. In openSUSE there is nothing blocking said window manager or other package from being included in openSUSE.
The other major reason for creating a derivative is to meet the specific needs of a smaller sub set of people it should be noted that there is a openSUSE edu derivative focused on education but there is nothing stopping you installing those packages on a regular openSUSE install. There is also nothing stopping and it is quite easy to package and ship applications that only a small number of people care about / use with openSUSE meaning they are more redaly pushed into the main openSUSE repo. This is evidant by the number of packages that have been added to openSUSE since the take up of open build service (Sorry couldn't pull up the stats in 2 mins but i read that somewhere)
It is also possible to use the SUSE Studio tool as mentioned in other answers to create your own custom openSUSE installer if the default doesn't have some of the packages that you want.