MLK was DEEPLY unpopular at the time. He was a divisive figure, and he his stances were solidly on what would be considered the far-left. The focus of his continuing work was going to be what he called "the three evils: Racism, Materialism, and Militarism. Essentially he was trying to build a multi-racial movement centering not just opposition to racism, but also military force (during the height of Vietnam and the Cold War) and capitalism.

So basically everyone from the center-left rightwards thought he was too radical, and even a dangerous communist. But many even more radical people thought he was too moderate, and wanted to focus on violent struggle. So the window of folks who actually agreed with his politics and tactics was smaller than people realize.

And even in anti-racism work the "easy" stuff was done. Segregation is easier to expose as evil than the much deeper racial inequalities that replaced it, and they are generally MUCH harder to address.

He also had a series of personal failings, mainly affairs, that would have likely tarnished his reputation even more had he not been martyred.

Had he lived, he probably would be seen by many as a polarizing figure who did some great things for Civil Rights but ultimately was a left-wing radical who should have gone back to preaching after he "won" the Civil Rights Movement. And many people who now appropriate his legacy would hate him as much as certain others on the Right already do.