I also used factory repo - and the explanation “why” is very simple. Always if i want to add some package that does not show up in my yast i go to software.opensuse.org and search for it there and do 1-click install
If You write “workbench” the main button for 1-click install is giving you the factory repo instead of 13.1
When I installed the version from server:database, it only required one additional package, which is also contained in server:database.
So add that repo to your system and the dependencies should get resolved automatically.
The easiest thing is to just click on the 1-click install on that search page, this will add the repo to your system and install mysql-workbench.