I disagree to a point - the advantage of a 20th century setting over random fantasy (apart from canon D&D worlds) is that there is less explanation. apart from D&D, where you can be reasonably sure that most players are familiar with most of the settings, fantasy, sci fi, et.c requires a lot of setup.
People already know what a phone, a shotgun, and an insane asylum are.
As to not being a good pickup game...
A CoC game that might turn into a campaign - where you would have "investigators" - is hard to play as a pickup game. A CoC game where everyone who happened to be on a scene at this minute are the characters is much easier. In other words, rather than have the characters choose
to be in the scenario, begin at the point where the action hits. All characters are equally unprepared, all characters are pregens, so the GM can make sure that any necessary skills are in there somewhere. It limits you, but it also makes people who are less conversant with the world feel it less.
Convention games are my specialty, and for many years, I was the
GM hose games sold out first.
...After all the D&D games, anyway.