Well you don’t put golden handcuffs on a guy who made a poor product even if he comes in on time and under budget.
Like Bryancd ,I think Rian came up with something original enough that Lucasfilm leapt on it. And I would guess thats totally the only reason he's passed on doing IX. He just could not wait to get started on his vision, which will probably shoot straight after.
Remember he is a huge fan of Star Wars and like the rest of us he has probably been mulling around decades of ideas about what he would like to see within that universe, who hasn't? Writing VIII probably solidified those concepts, but he probably felt they wouldn't work as well yet if they were directly connected to Skywalker timelines.
Remember there is this massive "unknown region" that has appeared in the SW galaxy and that leaves the potential for the evolution of many ,many new storylines and characters. Everybody must have dreamed about doing something like this, but freeing it from the old Empire/ New Order/ Jedi/ Sith staples seems the best way to go so that the stories after his trilogy link with the next series.
Personally I am now fairly desparate to know just exactly what he has done with VIII. Happily all the important story elements don't really seem to have leaked at all, though there is a ton of speculation . And the trailer "spoilers" seem to have been kept well under control ,you can guess at a couple of likely outcomes but really the bulk of the mysteries are really well hidden so far. I can usually guess with a reasonable degree of accuracy from what is revealed in the run up but this time I'm pretty much in the dark about most of it.
But Lucasfilm and Disney would NEVER have dared to make the announcement about Rians new trilogy if they were not HUGELY confident "The Last Jedi" was going to be a massive hit with the bulk of the SW audience.