Because using the backport repository means that all other packages are used from the back ports, too. And this makes it impossible to use a stable OS base.
That isn't quite true. The default pinning of the backports repo is 100 and the standard repo is 500. So, none of the backports packages should be installed unless you pin them higher or specifically install that version. When people install the backports kernel using the button in omv-extras, they will only get the backports kernel, headers, and firmware.