Why can't you edit the keyword when re-writing an article?

Well, apart from the fact that editing the keyword would allow users to write three articles with one credit if so they desired (with the limit of two re-writes), there is a good reason for it:

You don’t want to switch your strategy half-way through the creation process. You should do your keyword research and content strategy BEFORE you input the topic into CaS.

This limitation helps you get organized and plan ahead.