For me it's more that you can't edit a pdf file easily. You upload it and people who download it can edit it, take images out of it etc easily and adapt it to how they organise their comic collection.
RAR/ZIP/CBR/CBZ files etc can be downloaded, converted to rar/zip files, unzipped, pages added to them, pages deleted etc etc. Just much more easier.
Also in past PDF files tended to be lower quality than cbr/cbz comics.