While I cannot comment on posting quality, I will say that way, way back in the days of 2004-2006 Atlas was at least a lot more ideologically balanced from what I hear, possibly even marginally right of center back in those days.
"where things went wrong" is hard, if not impossible to pin point of course, not to mention the old posters from way back then also have posted that things were far from perfect at the time either.
Being that this is primarily an American forum, you could probably pin the reaction to favor mainly left-wing politics from the emergence of my generation that remembers watching our parents lose their jobs in the 2009 recession. We were forced to mature early and wonder why all these bad things were happening, which led to natural skepticism of such institutions as banks and other economic and political institutions. We’re also the first generation to grow up without remembering a time before the Internet. That would time more or less perfectly with this generation maturing and becoming active on the Internet, as a younger and more left-wing youth becomes politically active.