According to the Wiki for Riddick:
For 5 years Riddick has been neglecting the Necromongers as Lord
Marshal and has yet to take the sacred oath or lead them to the
Underverse. Several assassins try to take him out by orders of
Necromongers commanders, but all of them fail. Seeing that he has let
his animal side slip past, he orders Commander Vaako to take him to
his home planet. When Riddick arrives on a planet he finds out it is
Not-Furya, and the Necromongers betray him trying to kill him...
So it seems that despite being leader of the necromongers, he was too inactive in the role and they tired of him.
This is what the director, David Twohy, was aiming for as stated in this interview:
The first sequence is about 10 to 15 minutes of him alone on this
planet, with no dialogue and just minimal voiceover. And then he goes
into a... hibernation. Everything on this planet has tooth and claw.
Everything is trying to kill him. So he basically entombs himself, to
keep the predators away from him.
And as he falls asleep in his tomb, we flash back to the Necromonger
world. We get the sense that while it's great to be King, you have to
deal with the assassins outside your room. Sometimes the assassins in
your bed, and all the betrayals that take place there. With our
limited resources, I was able to do I think a pretty good [job] of
that in the movie.