...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
RSS is more simple, that's where its strength is. Atom is better defined, yes, but that's the problem: they made it easy to write a very complex feed when at the end of the day you want a simplified summary.
Why do so many people prefer RSS ? It's easy and gets the job done. You can edit it by hand; not so easy with Atom. Have you ever tried to write a feed reader ?
IMHO, the only thing Atom did good was multiple enclosures.
Why do some websites offer two feeds, RSS and Atom ? Because they can and because some ancient feed reader may not support Atom.