LW is mostly pure-text with no images except for occasional graphs. Why is that so?
Why shouldn't it be?
I am not implying that it should, but to answer your question, because limits on accepted forms of expression are not necessarily a good thing. Not necessarily a bad thing, either.
People already mentioned some pros (e.g. graphs and such help cross the inferential distance) and cons (e.g. images break the mental flow of some people).
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.