I asked this yesterday, should despotic governments be able to kill critics with impunity?
Trump and his family business have deep connections with Saudis and the royal family. He doesn't want to upset this relationship or prevent future Saudis from buying his million dollar apartments and properties.
https://www.cbsnews.com/news/trump-and-saudi-arabia-deep-business-ties-spark-new-scrutiny/
We should never (in my opinion) be in the business of supplying despotic leaders with arms. If they go to China/Russia instead, at least we're not profiting off their war machine that is killing thousands in Yemen.
https://www.nytimes.com/2018/03/22/us/politics/us-arms-sales-saudi-arabia-.html
I think a boycott of SA oil would be a good thing but of course Donnie will never do that because of his business's close relationship with Saudis and his close personal relationship to the royal family.