Why is it important to focus attention on one thing at a time? One impediment?
Well, there are many reasons. But let's take a few. And others may add other reasons in the comments section.
First, get something completed. So often we try to do "everything" and nothing gets done.
Second, we need fast feedback. For example, sometimes our "improvement" is a stupid idea. Only by limiting the number of changes can we begin to see how stupid we are. Or how brilliant (and maybe share the idea with the next time).
Second, we need fast feedback. (sic) We want improvement now, not in 6 months. (A related reason is the impact on team motivation.)
Third, to see better. We have kind of already said this. But let's expand a bit. In the Gemba (the team room) it is difficult to see specifically what is working and specifically what isn't. And it is very difficult to see through the tangle of inter-connections to what the second impediment is.
Only by doing the top impediment and then seeing the results, can we then decide what the next top impediment is. (Cf TOC.) Often, after we make a change, the next top impediment is in an area totally unexpected.
Fourth, blindness and fear. One example: We naturally want to think that the top impediment is in an area where we are competent to fix it. So, naturally we see those impediments and we ignore the others. (We are blind, and at the same time, we fear getting into, for example, "people issues" where those dreaded "feelings" might get involved. Or, some of us may fear getting into SCM or TDD or whatever.)
So, we recommend not taking a waterfall approach to impediments. But instead take a lean-agile approach to impediments.