Not surprising.
Nixon was widely popular until Watergate broke. Remember, he steamrolled McGovern in his re-election bid. Meanwhile, George W.'s poor second-term numbers were offset by the huge approval ratings he had in his first term (9/11).
Obama, on the other hand, was the most polarizing president to date. You either worshipped him or you hated him. He squandered the good will he had at the start of his first term and could never re-earn that trust because he was constantly fighting against the Republicans that the people sent to Congress in 2010 (and who arguably helped restore the economy he had a hand in wrecking).