Glad you asked! We started off filling it out for the popular startups, which eventually created the desire for others to join in and do it themselves (hence the rebranding from Leanstack.io to StackShare).
Regarding your suggestion, every page has this tab: http://stackshare.io/stackshare/stackshare/details. This feature was released today so not many stacks have the details just yet. But we'll be highlighting companies every week and having them fill this out, so stay tuned! Also, feel free to share your stack :)
On another note, it would be great to see a brief description about why the company uses a particular technology.