I'd go with laziness and lack of overt demand. I know that people love graphs and images, but I don't especially feel the need when writing something, and it's additional work (one has to make the image somehow, name it, upload it somewhere, create special image syntax, make sure it's not too big that it'll spill out of the narrow column allotted articles etc). I can barely bring myself to include images for my own little statistical essays, though I've noticed that my more popular essays seem to include more images.