Has someone said they don’t want to add it?
A matrix can be implemented on top of the custom rule system, but not vice versa. The rule system is much more powerful and flexible. As far as I have understood there were two options: either a matrix with a GUI and then a backend dealing with the router and DRC, where the backend is probably the more complex part; or a rule language and more complex backend. It was decided that the better rule system was needed anyway. Now we already have the backend. Adding a matrix means adding a GUI which creates simple rules for the existing backend. So, now when we already have the backend, implementing a matrix for ease of use is almost trivial compared to implementing a matrix GUI + a custom backend.
But this doesn’t happen for v6 because the major features have been planned and decided, the feature freeze has gone already. This is necessary to keep even remote resemblance of a release schedule – otherwise we would never see the final v6 release. So, it can happen in v7 at the earliest.