Murdering dissidents while they are in other countries doesn't happen all of the time, no. That's why it's news.
My guess is this case was specifically targeted by media influencing government agencies to exert pressure on SA.
The case itself is also almost a perfect fit - a good link to the USA and English speaking world, very solid evidence collected by Turkey, etc.
I suspect there are also some where there is reasonable evidence a foreign state was involved, but the victim simply wasn't sufficiently noteworthy for the media to care, and the case not being worthy of getting high level politicians involved in. For a state, using the brief time the president has talking to another state president to discuss a missing person rather than finalizing a new trade deal is a bad move.
We're hearing more about Khashoggi because he was a journalist, I suppose.
If these things happened all of the time they wouldn't be reported in national papers. A mugging in San Francisco isn't in the Washington Post.
He was a columnist for The Washington Post. He was known for having interviewed Osama Bin Laden a couple times.
News was slow and after the Chinese trade imbalance/conflict (finally someone stepped up to address it), so it looked like there was news to be had against another objectively-bad-government. Even if just to see what Trump might do to resolve (or sabotage) our uncomfortably close relationship. Nope, he just tried to promote it like the moron he is, in context of the assassination.
I really wish he woulda dumped another of these 19th century (depending on when they became stable) strategic allies, as I'm certain there's 0 chance the next POTUS will.