I think it's also somewhat hardware based as well. As I've found it, it requires a pretty hefty CPU and probably more memory as well. If it was easy to do (with just software) I'd suspect it would have been added years ago.
Keep this in mind, the P3 has the ability to follow someone right now. The examples that DJI show are not real world demos as it shows people running around and being followed by the P4. Well, you still need to keep the controller within range unless you want the P4 to try to fly back home when the battery goes low. What the P4 really adds is it's ability to keep a object in frame within a confined area. But how many times will this be needed? You can so this manually with a camera, the drone just gives an over-head view. Ever thought about flying your drone at a baseball of football game? People will love that! So really, how often would the Active Tracking be used.
Last comment... Active Tracking is a nice feature. I someone wants the P4 because it has this, great! I'd love to be one of those people. Additional options are always good.