I think he's just snowballed in popularity over the years because he's good at reading these scripts in a way that sounds both smart and fun. The different channels focus on different styles and subject areas, letting you pick and choose what kind of thing you're interested in.

I rather like his "Decoding the Unknown" channel, where he gets scripts debunking various paranormal or otherwise mysterious events and he reads them for the first time as it's being recorded, taking lots of opportunities to interject his own theories and speculation and just generally rag on the concept of the paranormal as he goes.