All due respect, I would disagree with your findings. You yourself are not an addon developer so I'm not quite sure how much you can relate to this, but X-Plane is becoming easier and easier to develop add-ons for, more so than other flight simulators. Applications like W.E.D. and PlaneMaker are becoming increasingly versatile. The main reason there has generally been more add-on development in the past for other simulators is simply due to the larger community following. However, as X-Plane drastically increases in popularity, add-on development has also dramatically increased.
There is nothing to explain. Dev tools have been constantly improving. It is a process. It is not Laminar Research's job to support SDK tailored to a specific development team, as there are already many developers on the XP side using different means of creation. Obviously the hope is that more developers are able to jump to X-Plane, but frankly the L.R. team can only do so much. Perhaps after the release of 11.30 there could be more focus on what you discuss, but I couldn't say.