I absolutely do not understand why my edit was rejected
The answer is in the rejection reason: the edit was too radical. But it seems your real question is something like "okay, it's a big change, but it adds useful information, why reject such edits?" And you also add "Wiki means you edit existing content where appropriate".
And the answer to that is that this site is not a wiki.
It has some wiki-like aspects - in particular, these kinds of stuff are meant to be community-edited (and also notice their names):
- "Tag wikis" - the short and long description of tags
- Answers marked as "community wiki"
And we also facilitate a very simple mechanism for editing regular questions and answers, similar to wiki sites. But you are not supposed to radically edit them, as you should in a wiki site - instead, this is intended for fixing formatting, typos, small mistakes, etc. That's just the way we work.
"But why is that the way you work?" you may ask - well, because the main goal of this site is not to create pages with comprehensive information, as wikis try to do. The main goal is to answer the questions asked - preferably, as directly as possible. This is also the reason for encouraging multiple answers and with the ability to vote between them - because we want to make sure the thing that was most helpful to others appears right there at the top of the page, to provide an answer as quickly as possible, without reading tons of information.
Now, this does not mean you should avoid writing long, comprehensive answers - just keep in mind that you write an answer, not a wiki page. And if you look at existing answers and find them lacking, the correct approach is to post an alternative. This also plays well with our reputation system, as fbueckert has written above - by keeping answers separate, not only are they voted on separately, they also affect the reputation of their owners separately.