Why are contributions to the education of the broader public not valued much within academia?
Scientists not only need to discover new knowledge but also have to share the knowledge to others. For the former, we have things like universities, collaborate groups and journals to take charge. For the latter, we have various forms from teaching, developing and maintaining softwares to writing or translating books, writing and editing in Wikipedia, answering (and asking) in SE, joining an academic club, working on a academic-related project, blogging, etc.


I think this is not correct. Discovering new knowledge is the actual research (valued on all levels), "sharing to others" is writing publications (valued so much that the common criticism is rather that publications count too much).

What you mean is a different type of knowledge dissemination, basically the education of the broader public. This is indeed currently not widely valued, but I am not convinced that this is a fault of the system. Essentially, when an university hires a researcher, they want a researcher. They are specifically looking for somebody that generates new knowledge, not somebody that is good at breaking down this new knowledge for the layman (this would be a science journalist, or somesuch), nor for somebody that mainly collects and summarizes the knowledge on Wikipedia or Stack Exchange. 

Hence, the answer to your question:


  When will contribution to community be valued?


If you understand this as "when will my contributions to community be comparatively valuable for applying to a research position as research results", then the answer will likely be "never".