My strong impression at the time was that the primary impetus for establishing Atom was that Dave Winer was abrasive and opinionated, and some people just didn't like him very much.
FWIW, https://en.wikipedia.org/wiki/History_of_web_syndication_tec... notes that Atom was a side-effect of RSS being frozen. Also FWIW, Dave gave Atom (then Echo) a tentative early endorsement:
"If and when it reaches closure, I will recommend to UserLand that they support the format, both for input and output, and I will help to the extent I can to write drivers for the software. I will continue to answer questions for the people working on Echo, and offer my opinion when it appears to be welcome, but will step back when it is not." — http://backend.userland.com/2003/06/26
And, true to his word, Dave added support for Atom in UserLand products in 2007.