Because the FBI is focused on Jihadists—a much greater threat than right-wing radicals—they have been able to stop potential attacks by Muslim radicals before they occur.
Here’s some of them:
https://en.wikipedia.org/wiki/List_of_unsuccessful_terrorist_plots_in_the_United_States_post-9/11
...And, the list leaves a bunch of terrorist attacks out. I guess they just included the ones they could think of off the tops of their pointy heads.
I don't think the FBI have been slackers in monitoring US racialist groups.
It's more a matter of Islamist attacks involving more than one person, even if it's just one person lecturing another about jihad. There's a trail of communications and connections the feds can uncover.
With non-Muslim terrorists it's more a matter of "lone wolf" attacks. There isn't even the flimsiest coordination involved, so it's much harder to discover plans and prevent attacks.
What will probably happen now, is the feds will look at traffic on racist sites in the same way they now look at Islamist chatter, if they're not doing it already.