Because people use those languages for configuration, and Github is hugely popular for storing dotfiles. Configs are forked, tweaked, and reuploaded a lot more than "actual" programs, because popular starter packs and so on get huge followings, and there is a much smaller barrier to getting started playing around with them. Meanwhile, they're not really that complex, and most have other sources of question resolution (inbuilt docs, online communities often inside GH issues), so not much need to go to S/O.