This question is different from this one here. I'm asking "Why". And, putting a tag of 'feature request' to improve this site.
It has happened with me a couple of times. I answer the question and then after some hours or even days, the question is 'entirely' changed invalidating my answer. This might have happened with new users here as they are advised to improve their question (for better clarity) by seniors on this site, which is absolutely fine. But then, there should be some system that notifies all the answerers so that they can improve their answers as well (depending on the change).
One such example is here. This question first asked 'object' and its verb. Now which object was not so clear as the picture first showed a girl, a bag, infusion set, peripheral venous catheter etc. Since asked by a layperson, I answered it 'bag' and 'intravenous' or 'infusion'.
But then, after a day, the OP comes with a new image now clarifying what exactly he wants. I'd have answered 'IV bag' at first, if this image could have been in the original question.
This is okay in normal cases, but then, in some unfortunate cases, changing the question invalidates the answer completely. And then, it starts attracting downvoting.
There should be a system (and it does not take much pain) wherein the 'answerers' are notified whenever the question is being changed. 'Edit' in questions are not that frequent but whenever there are, they are the crucial ones.