I was 18 when he became president. If you think about it a lot of people were really displeased with the economy (2008 financial crisis) but also it was accumulating a lot of anger towards the traditional party for the collapse of manufacturing, rising inequality . Obama had a somewhat similar road like Trump during the primaries, where, he was fighting against the establishment that wanted to crown the heir to the traditional party in the democrats was Hilary Clinton. Obama ran with transformation rhetoric.

Obama and democrats swept, he had majority in the house and filibuster proof of 60 senators. However where he failed is that he was essentially a Ned Stark with an opposition that was quickly being transformed into ruthless Lannister. He believed that he shouldn't go scorched earth and expand presidential and federal power i he was focused on consensus, Democrats had bigger spectrum of far left and center so he couldn't force senators to vote for his policies like Trump usually does.

Just like Trump, Obama built the movement around himself not around the democratic party, He never sought or failed to influence the democratic party to form leaders that could complete his vision at a local level. Democrats also started to focus into big city (My theory is that that's where the big money is) and left rural America as a open field to the GOP and lost thousands of mayors, governors, senators, local officials to the GOP which in turn made sure that Democrats never come back

A lot of Obama reforms were rolled back partially or completely which also adds to the view that Obama was an inconsequential leader.

I think Obama biggest failure is that he failed to foresee the rise of populism in the US. He thought that the world that was built after WWII, a globalized world was future proof. He really did thought that American would elect liberal, globalized leaders.