Is it that his videos are relatively short and digestible in 5 min, and have some off-the-beaten path / uncommon / eye catchy subjects?
Does he just make the right combination of detail and high level takeaway?
Is it that he actually goes to the places to get live footage (versus those Youtube regurgitaters who explain a topic never having left their desk, with 200 ADD-style stock footage clips in a single video)?
Is it that he frames each video with a larger more philosophical question?
Is it because he has a friendly face?
I watch a channel with some quirky British/London guys on weird things about the city (https://www.youtube.com/watch?v=cTLCfl01zuE, https://www.youtube.com/watch?v=Na9iO_HEe14) but it has never taken off like Tom Scott (-- and this is amusing, despite Tom Scott having initially been the guy who designed their intro clip). Maybe it's just too local (no so-what for me) to feel like it's broadly interesting, or they don't lead out to the bigger theme so-what?
Maybe there's just some confluence of x-factors that propel certain people to fame.