Timing, mostly! It's easy to forget that most of these guys have been doing it for years - for instance, PewDiePie (most subscribed on YouTube for those less familiar) started in 2010 so he's had plenty of time to grow - not to mention the fact that in 2010, Let's Players weren't as much of a "thing" as they are now, so he was able to take the market by storm before folks like Markiplier, JackSepticEye or Game Grumps had even started! They're all 2012, I believe.
Man, I probably sound so stalker-ish, but I'm honestly just good at remembering numbers. ;-;
Now, it really would have to be about the content, more than anything. That and a metric tonne of luck.
I mean, you should care about what you're producing anyway, but because of how saturated YouTube has become in more or less every genre known to humankind, you have to work 100x harder to be seen and have people go
hot diggety dang that person is mad cool better clickety-click that red button hoo boy