Availability helps But Freeform Games larps are very much aimed at the non-gamer market (they call them "murder mysteries" and target the office party market), so they have very good information on logistics and running the game. They also have accessible themes (pirates, gangsters, westerns), so its one of the default recommendations to people.
Checking, Donna's stuff ("Graduation Day" and "Down and out in Middenburg") gets run for new players, not necessarily by new GM's. And that's a combination of availability, accessible themes and light content (they're both fantasy comedies).
Recently I've also been recommending "on Display" to new GMs, since its pretty easy for new players and has a low GM workload. I'm waiting to see if it actually gets run again by anyone here though.
(Chimera's archives are currently offline for a website update, otherwise I'd be trawling for news GM's running other people's games to get some more data).