He didn't leave. He was fired for unknown reasons.

But according to Zuck not due to "controversial political views" (Palmer is a passionate libertarian who supported Donald Trump, several Republican senators and the Republican party both in California and Washington.), but that's quite interesting because after word got out he supported Trump he was silently stripped of management responsibilities at Oculus ("In the wake of the revelation, Luckey’s role at Facebook was quietly reduced." -- https://www.theguardian.com/technology/2017/mar/31/palmer-lu... )

Also was a big disappointment for me personally, I got interested into Oculus due to his work and that of Carmack.