It's very strange, CBS has a habit of reinventing the wheel, and constantly changing and shifting their focus. The programme is 3rd because it's never recovered from the Killian documents scandal nor has it ever settled - and when it does (under Couric and Co, maybe not as successfully, and certainly with Scott Pelley) they change and change everything.
Maybe they need to forget Walter Cronkite and his day and focus on the present day, this obsession with the past seems to be their biggest issue (when do they NOT mention Cronkite in an anchor shuffle, or in a history item?).
It goes back much farther than the Dan Rather Bush 43 fiasco in 2004/2005. CBS has been in perennial third place going back to the late 80s. Viewers never liked Rather, though in his defense, and as Rather put it himself, anyone who followed Walter Cronkite was going to get his head blown off. The huge affiliation swap in the mid 90s also left CBS with a weaker portfolio of stations, which definitely hurt them as well.
Going back to more recent times, even when CBS got it right during the Scott Pelley era, the newscast remained in a distant third. He pulled the ratings up a little bit, and it was a very quality newscast, but viewers here have deeply entrenched habits. And in typical CBS self-sabotage fashion, internal politics forced another format and anchor change.