Maliki wanted to be a sectarian leader, Obama wisely let him fail.

Maliki used his paranoia to widen the sectarian mistrust and fuel the ISIS invasion. He made it impossible to have an inclusive government so the outcome is predictable.

If Obama had acted then, the situation would be worse.  To help Maliki would have tarred USA with the same brush, to engender hatred against oppressors within Sunni community.

Instead, Obama wisely let him fail and the unintended consequence was the ISIS advance happened faster than expected. But ISIS would have invaded later on it was inevitable. 

By not propping Maliki up, Obama used the leverage of the situation to get a better deal from the Iraqi government. Now the Iraqi government, desperate to win, will need to accede to US demands that they would have ignored months ago.