Is there any reason why www.blender.stackexchange.com not implemented the login by BlenderID?
The Stack Exchange website is not affiliated with the Blender Foundation, it is run by Stack Exchange Inc. which hosts a variety of sites each run by a community of volunteers and their moderators. Since it is a community website hosted by a company that has nothing to do with the Blender Foundation, there is no option to use Blender ID. This login method is used for the official websites of the Blender project and selected websites such as blenderartists.org.
But in hypothetical case is easy to use/implement BlenderID in other website? If they want maybe?
I don’t know since I have never worked on the implementation or infrastructure for Blender ID. Additionally, whether something is “easy” is difficult to say, as this very much depends on the experience of the people involved and the circumstances of this hypothetical project. You would definitely need to coordinate with the Blender Foundation and get their stamp of approval. Most likely they won’t provide the Blender ID / OAuth service for just anyone, but this question is best answered by people actually involved in running the infrastructure.
Perhaps if @fsiddi has time, he could explain how this works.
Blender ID integration is only available upon request. This feature is mostly used by official Blender websites (Open Data, Conference, Cloud, etc.) and some community websites.