"Is it that professors don't want to learn HTML and maintain simple course webpages?"

Here is your answer. In particular in the humanities. I had many professors who while they hated the software, were bad at using it and used it basically as a shared folder for powerpoint presentations, couldn't figure out a better solution. They felt that they were not skilled enough at technology to solve their problem on their own. Unable (and or unwilling) to create their own solution they bashed through with the software.