Obama made the n00b mistake of thinking that Republicans objected to Liberal policies based on some deeply held principles and not because calling people commies was a convenient political scapegoat for the last 60 years.
As a result, he managed to pursue half-measures when they Dems had the supermajority, that didn’t really excite the base much. By the time he wised up the Republicans already had the house.
Also, Obama was just terrible for the Dem electoral machine. You can still see that today, where the Dems still suck at election campaigning.
Edit: imo the biggest problem politically this country has I The gerrymandering from 2010. So many congressmen sit in uncompetitive districts that Republicans could hold the house running 300 Todd Akins except for maybe this year.
Luckily this should improve as there’s almost no way republicans have as much control of state houses in 2021.