That's because he is personable and likable but his policies were not.
His policies were horrid. Obamacare flop, Rise of ISIS due to a weak and often times conflicting FP. Anemic growth after 8 years of stimulus. Raising the national debt more than any other president, etc etc.
His popularity doesn't translate into good policy, and that is what Americans were rejecting.
I don't know how it could be more obvious. The problem is his party is slow to accept it. They really think they can go forward with the same basic policies after Obama is gone.
I didn't think Obama would go two terms because I thought enough people would realize that likability in a president is great, so long as they were an effective leader and had a sound agenda. And Barack was or had neither.
In foreign policy, the world didn't square with his vision, so he basically just checked out and withdrew from it. The results have been predictable. His main gig was domestic and social policy---note how when local racial issues sprung up he couldn't resist putting on his community organizer hat.
Anytime a Trayvon/Zimmerman type issue sprung up, you knew the Obama-weigh-in was coming.
Progressives and SJW's loved him for it [especially, because he unfailingly came down on the politically correct side] but he was unwittingly driving away *non-racist* white voters and opening the door for Donald J. Trump.
But in 2012, republicans failed to capitalize on it and ran a weak candidate in Mittens Romney.
Trump is the 'Obama antidote', in a certain sense. And I get the idea future historians will criticize us for not seeing him coming.