Two points: because his leadership begins to suffer as he ends up having few chips to trade with Congress to get things done; and because a fresh round of Presidential candidates rise up - sometimes even within his own party - either to dismiss or deny his efforts at meaningful legacy bills during those years. Reagan seemed to suffer from it, as had Eisenhower and Bush the Lesser.
But that's not always the case. In each of those Presidencies, a lot had to do with the failures of those administrations coming back to haunt each one (Eisenhower's failures in foreign policy, Reagan's corrupt administration collapsing under the scandal of Iran/Contra, Bush the Lesser's inept rule and bad wars). Effective Presidents entering this phase can still craft out legislation and effective policy changes at will, even with an opposing Congress: Clinton had success during his last two years even with the Lewinsky impeachment vote, and now it's looking like Obama might with opening relations with Cuba and Iran.
In particular, Obama seems freed from the obligations of "playing nice" with an opposition party of the Republicans that obstructed him from Day One. In the past few months, ever since he dissed Congress during his SOTU, Obama has basically been slapping the taste out of the Republicans' mouths.
Obama, as the saying now goes, has zero f-cks left to give.
Leading to this moment at the White House Correspondents' Dinner:
Makes me think of the cool memes we got when he started his Presidency...