...instead of using the Atom syndication format?
Atom is a well-defined, general-purpose XML syndication format. RSS is fractured into four different versions. All the m
The 'why' was fairly well answered, but I would suggest going forward that developers only implement Atom on sites. There's no reason to have multiple formats for a site available and any modern feed client can now parse Atom feeds.
Atom has quite a few technical advantages over RSS and is being widely supported and utilized by major companies such as Google and Microsoft.
As for branding, I don't really care what acronym is used. I think the universal orange broadcast icon or the word "feed" is what people care about. Despite the proliferation of RSS and Atom feeds, I'd say the average web user still has no idea what they are. Looking at it in that way, the whole concept of syndication feeds is still in its infancy.