One reason: a PDF feels like a "finished publication". While HTML can be dynamically updated, a PDF is the equivalent of something "going to the press".

Another reason: for whatever reason, if you're selling your content a PDF has more intrinsic value for the end customer. It becomes "a thing" instead of just being "content that should be free". It's very odd. People like the idea of owning digital goods, but only as long as they can download them and store them on their own hard drive.